Well, nobody else answered you, so I'll give it a try. I'm no expert, but am thinking of converting my attic as well (therefore, I have been doing some research). First off, your local codes will be the determining factor. However, you don't have enough headroom (you need 7' 6" over 50 % of the floor space). Second, 2x4's are not floor joists. A stronger floor would have to be built (thereby cutting more into your headroom). Third, when I hear 2x4's in an attic, I think trusses. If you have trusses, you can't modify them. Your will need a whole new roof. It would be cheaper to build out than up. Do some searching on other home improvement boards and you'll find more info.