So, I've developed a mod for 1.6.4 Forge that essentially lets you convert 1 block of wool into 4 string. I'm PRETTY sure that vanilla minecraft cannot have this because it adds a way to invert recipes, which, as far as I know, does not exist in vanilla minecraft, and for a very good reason that I will not seek out. This inversion is due to the vanilla recipe for 4 string into 1 wool existing.
however, if you are interested, there are some incentives for downloading this mod.
This mod is for you if you are:
a person on peaceful without a mineshaft
a person with an abundance of wool and a lack of string
a person with a lack of wool and an abundance of string
a person that wants to temporarily store string as wool but have it again later
a person that can't decide whether to convert their hard earned string into wool or keep it for later use
Simple to apply. just add the zip to the mods folder (requires forge for 1.6.4), put a wool into the crafting menu (or 4 string) and bam! You got 4 string/ 1 wool.