The problem is that the 2' x 4' rafters aren't deep enough for adequate
insulation. So here are my choices, and I need some advice from someone
knowedgable.

assuming you wouldn't be asking this question if you had a flat attic you could insulate, if you want R38 in the roof, you will need 12" of fiberglass PLUS a ventilation space. I would guress the cheapest way to attain that would be to build down the existing rafters with another 2x4 which could be hung with plywood gussets. this system would reduce thermal bridging which is a good thing. then install r 38 batts. foam would be a nice way to go but it is expensive in comparison to fiberglass. if you lay foam on top of the roof, you will need to add another layer of plywood on top of that (though if it's steel, i suppose you could just add some firring.).

BTW, i once owned a house that had 2x4 walls. I firred them out with
2x2's and insulated with r 21 batts. was quite easy and worth it i
thought.