How far can i span with a 2"x4" & meet 40 lb s.f.Floor Joists Spans
2x4's are NEVER recommended for floor joists. 2x4 ceiling joists can be used with a span of 7' to 9', depending on the species of wood.
Never use 2 X 4's for floor joist. You'll regret it later when the floor gives way. I recommend 2X8 at the least. It depends on what your building, if its just a shed or a house. A house is 2X10 or 2x12, preferably 2x12's on 16" center.