The socketing quests always gives the max amount of sockets, so in your case, 5 (according to arreat summit).
Keep in mind, however, that if you change the quality of an item (cracked -> normal, etc), then the ilvl of the item is set to 1 I believe, and the max amount of sockets will be less than you might expect.
if it helps you have a better chance of getting 4os on your current bow than finding a +3 gm 4os bow. I guess the question is are the odds worth losing [insert other ingredients here]?
In fact, it looks like your odds of four sockets are pretty good, considering what's been happening to me. I've cubed 5 sets of archon plate so far trying to get 3 sockets, and gotten four 4 sockets and one 1 socket. But if that's anything to go by, all it tells you is that getting the right number is a pain...