2 things to think about:
1) 6' and 4' measured *where*. If that is wall height and you will have trusses or rafters sitting on top of those walls, ok; if that is total structure height, though, remember that the roof itself and the trusses or rafters will be stickin' down lower, probably 6-12" depending how you're building it.
2) what roofing material are you going to use. If metal or plastic, no problem; if shingles on plywood, though, remember that roofing nails are going to be sticking out all underneath the plywood, and you do NOT want your head anywhere near that (like, you want at LEAST six inches to a foot clearance all over) or I promise you you'll be headed to the ER to get serious scalp lacerations stitched up sooner or later (prolly sooner). Those things are SHARP (and numerous)
Good luck, have fun,
Pat