2x4 roof structure for garage
 

The space above the garage is a 2x4 structure. I want to use this space for storage. Will it hold or do I need to reinforce

if its trusses, you can . but if its stick build, you will need reenforcement.

got any pics ?

This is a pretty common question on this forum. I suggest you search previous threads for a full discussion on load capacity of floors. Whether the 2x4's will hold depends on the span of the joists, the species of wood, the spacing of the joists, and the load per square foot of what you want to put up there.

Got any pictures.
Going to need far more info, your there were not.
Unless you plan on storing styrafoam, 2 X 4's are not going to hold much.

The span of the joists are about 20'. The material is 2x4 yellow pine on 24" center. Plan on storing just misc household items (xmas stuff, coolers, etc.) nothing heavier than I can pick up.

2X4's are going to sag, there in place to keep the rafters from spreading.
For that span you would need at least 2 X 8's.

yeah. they are probably sagging already. i have a 2 2x6's in my new garage that span 21', and they sag. if the rafters are in good shape, you "may" be able to put some (can't think of a good word for them) braces that go from the joists to the rafters to help hold some weight. my new garage has them, and they work, to a limit.

Assuming an allowable stress of 1200 psi (reasonable for yellow pine), and assuming the true dimensions of your lumber is 1.5 inches wide by 3.5 inches deep, and based on your description of 20 foot spans, 24 inches on center, your 2x4 joists can hold approximately 3 pounds per square foot based on bending capacity. This means you cannot store anything up there safely, in fact you should not even walk on the joists. As mentioned previously, those 2x4's are not there to support a floor, they are only there to prevent wall spreading, for which they are fine.