How much will an autographed guitar signed by 20 guitarist cost in the future.?

You should definitely put that in a case somewhere that it will stay in perfect condition! If I were you, I would post it up on ebay. I'm quite sure you could get a lot more than $4,000 right now off ebay. I guess you could hold onto it and sell it later (possibly making more) but I would be so paranoid about keeping something that expensive. I would rather sell it now for profit than risk it getting stolen/damaged.