You don't say what the span is, but I'd be leery of expecting your tubafour ceiling joists to hold much weight.
Not to mention that wrestling full sheets of ply, of any size, up to overhead storage is not my idea of efficient use of space and energy (mine), even it the rafters would hold them.
My shop (1930"s vintage) is made entirely of 2X4's also, with an 18' span, and other than the odd valuable plank or two stored overhead, and close to the walls where the joists will bear more weight, I long ago discounted the idea of overhead space for lumber storage.
I store most of my wood vertically leaning against one of the tall walls ... generally bought on a project by project basis.
Plywood (mostly offcuts) does better in my shop stacked on edge, also against a wall. I'd rather let the lumber yard store the full size sheets for me and I just pay for 'em when I pick'em up. Full size sheet goods get cut immediately into their component parts.
You have to accept that the inability to store much material is one of the compromises when working in a small, single story shop.
YMMV