Are you saying you have the enchantments on books to hand (in which case list us what you've got) or are you going to use an Enchantment table (in which case you can only get what the table offers)?
Personally, I maximise my armour as much as I can, starting with the best initial table enchantment I can get, then use books to add the others (I have a villager trading hall).
I don't bother with Thorns or Frost Walker - thorns might be good to help you in a fight but it also reduces armour wear. In terms of the protection enchantments, I believe the maximum level is capped so you don't need all your armour to have the various maximum Protections on them, but I tend to anyway, and I mix it so I have say Fire Pro on one, Projectile on another, Blast on another etc. I also put Unbreaking III on mine to help durability, although I understand it isn't as good as using it on a weapon or tool, but I have loads of U3 books, however if you put Mending on your armour, then you don't really need U3. Depends if you have any Mending books (only available from fishing, chests or villager trading). If you have a Mending book maybe Thorns would be a better prospect, as you could put the Mending book on the chest plate too to cancel out the Thorns wear...
The main extra ones to consider are on boots and helmet. Feather Falling IV good for heights if you get knocked off something, Aqua Affinity, Respiration III and Depth Strider III don't really help in fight situations (unless you are escaping/fighting in water, in which case the last two might help)
As far as Protection goes, I would use regular Protection on three pieces and Blast Protection on the fourth since maximizing Blast Protection is far more important considering how much damage explosions do, especially with armor penetration; in fact, it is the only way you can survive a close-range creeper explosion on Hard (in 1.6.4 I only wear two pieces with Protection IV and no helmet; the only other damage-reducing enchantment I use is Feather Falling. In my current modded 1.6.4 world I added Protection to my boots since I reduced the effectiveness of armor, giving me nearly the same damage reduction as in vanilla).
Aside from explosions and fall damage, other sources of damage really do not warrant any special enchantments; projectiles? Skeletons only deal up to 5 damage, which is reduced to a bit over 1 in unenchanted full diamond armor (armor penetration is all but negligible; you lose one armor point for every 4 points of damage; if you are not wearing full diamond it is more significant at one point per 2 points of damage, since diamond armor has +2 armor toughness). I would not even bother using shields against them even if it meant negating arrow damage (I have played a bit in 1.9+ just to see how much things actually changed, which is not much at all; I used the same armor and weapons I use in 1.6.4 except I added Blast Protection to my armor). The few mobs that do deal significantly more damage are either melee mobs, which only Protection reduces, or have explosive attacks.
Fire Resistance potions are far superior to Fire Protection - 100% damage immunity, including no armor wear; in any case, i only use them in the Nether, carrying around (and liberally using) a water bucket when caving. Also, a fun fact: Since 1.9 saturation restores half a heart every half second; fire damage (burning)? Half a heart every second - so you can out-regenerate fire damage by a factor of 2! Same for Poison I and even Wither II (note that only Protection reduces the latter two, it also reduces fire damage, if not the burn time).
The only times I have come even close to death recently are from close-range creeper explosions (more than one in a row; a creeper exploded and knocked me into another one, which left me at 3 hearts; there were more creepers and other mobs but they got blown up) and poisoning (while safely walled-in as in 1.6.4 you only regenerate half a heart every 4 seconds; even then you won't go down to half a heart unless you were already damaged).
Protection IV gives you an EPF (Enchantment Protection Factor) of 4 per piece, so 16 total for all four pieces of armor.
The game allows up to an EPF of 20 (capped), so Protection IV gives you the most all-around protection against everything, but it does not max out the EPF. The more damage specific enhancements give you double the protection for their targeted damage type (EPF 8 against projectile, blast or fire).
Three pieces of Protection IV will give you an EPF of 12 for all damage, and with Fire Protection IV on the last piece you will have the max EPF of 20 against fire damage. That's the best protection you will get against lava without potions (of course, potions of Fire Resistance are the absolute best).
If you tend to fly around with an Elytra and only wear three pieces of armor, you could use 2 pieces with Fire Protection IV and 1 with Protection IV and you would still have an EPF of 20 against fire/lava, but you would only have an EPF of 4 against other damage types.
Substitute Projectile Protection IV if your nemesis are the new strafing, aim-bot skeletons.
Substitute Blast Protection IV if you are getting blown up by creepers a lot.
With 2 pieces of Protection IV, 1 of Blast Protection IV and 1 of Fire Protection IV you would have an EPF of 16 against both fire and blast, but only EPF 8 against other damage.
So it's just a matter of deciding what you want to get protection from, and getting the appropriate enchantments.
Personally, I use all Protection IV diamond armor.
Personally, I use all Protection IV diamond armor.
If you play on Hard, or on Normal with less than full armor as I do, you certainly want to consider replacing one piece with Blast Protection:
Armor penetration = 20 - min(20 * 0.8, 73 / 4) = 4 armor points left (16% damage reduction; without the 80% maximum penetration you'd only have 1.75 points or 7% damage reduction left) = 61.32 damage after armor (3 times the fatal damage! You'll certainly die up to a few blocks away; with just half the maximum damage (close to what they deal if they just walk up to you while you are standing still) you'll get slightly over 20 damage after armor, so even Normal difficulty is a one-shot kill up to 1-2 blocks away)
With full Protection IV (an additional 64% damage reduction, acting after armor calculations) = 22 damage after armor (2 more than needed to kill you. Sure, you'll probably survive in most cases since this is absolute point-blank range (you must be standing in the exact same spot, which requires being in a 1x1 space due to entity collision) but I want to be 100% sure that they can never one-shot me or leave me near death)
With 3x Protection IV + 1x Blast Protection IV (an additional 80% damage reduction) = 12.26 damage after armor, a bit over 6 hearts (interestingly, about the same as the worst-case (due to randomized protection, which was removed in 1.9) that I can get on Normal difficulty in 1.6.4 so by simply upgrading armor I can offset all the difficulty "increase" from Normal on 1.6.4 to Hard in 1.9+)
Also, I have at least one anecdotal report (I have not tested it) that Protection no longer reduces fall damage as of 1.11.2, making Feather Falling even more vital (I could see why; 80% damage reduction from fall damage is quite extreme as you can consistently survive 103 block falls, prior to 1.9 it was a random 52-80%, guaranteeing survival from only 44 blocks. If you now only get protection from Feather Falling (12 EPF, 48%) it is only 41 blocks):
Unless they have made an update in 1.11.2 to armor enchantments, protection does not reduce fall damage. You can try an experiment for yourself to prove this, test if you die at 24 blocks fall distance with protection 4.
Yes, I always have Feather Falling IV on my boots. I didn't mention that because I was just referring to the enchants that are mutually exclusive, and FF can be added to boots regardless of other enchants.
I have survived more than one close encounter with a creeper with all four Protection IV armor pieces in place (hard difficulty), but I have no doubt that would not be the case if I had my Elytra on.
The Meaning of Life, the Universe, and Everything.
Wakefield, West Yorkshire, UK.
West Yorkshire, UK, Earth
My iron armour does have aqua infinity, feather falling plus depth strider (which is a something I can not live without) everything else I just leave to the enchanting table and a bit of luck I do try and get unbreaking III on pieces of armour but it's normally not something I think about, Just create, enchant and go!
I have plenty of Protection books finally !!! after some villager trading So that does look like priority for my armour.Protection IV
But as this is in my new survival world I didn't want to enchant my armor and mess anything up before having the best overall protection I can get.
Normally I would have just gone into creative and done some testing myself But I wasn't at home and sometime it's quicker to just ask