Just came to this thread, though Inga and I have exchanged some PM's on this issue and I won't repeat all that.
My recommendation for a 55g (4-foot tank) is a single-tube T5 fixture (as Mikaila mentioned in her last post), or a dual T8 48-inch, or a single T8 48-inch. I had the latter when I had a 55g some 25 years ago and managed, but you are limited to low and moderate light plants so don't expect most stem plants (aside from Pennywort) to do well. The T5 HO single tube gets you up to moderate which will grow the vast majority of plants. The dual T8 48-inch goes a notch up from that.
I would not suggest a dual T5 over this tank; I had such a fixture and tried it for a week over my 5-foot 115g and it was way too bright. I now have a dual T8 48-inch over this tank and the plants are thriving. But again, I do not even attempt high-light plants. I am more interested in keeping the light low for the sake of the fish than I am in propagating plants.
Single-tube T5 fixtures are hard to come by. Fosters&Smith has one, here: Aquarium Lighting for Planted & Reef Aquariums: Hagen GLO T5 HO Linear Fluorescent Fixtures
and they say it is clearance. As I've noted elsewhere, T5 was primarily designed for reef tanks to provide higher light intensity with fewer tubes than was possible using T8. The T5 is still aimed at marine setups, and one tube would obviously achieve little there. Now that manufacturers are moving into LED, I doubt we will see much development in single T8.