If you want the lights to sit right near the top of the tank, then 4 foot long lights will leave the ends of the tank with noticeably less light than the middle of the tank. If you can have the lights 10 inches or so above the top of the tank, the 4 foot lights should work ok.
Assuming you put the lights right at the top of the tank, you only need one T5HO bulb to light the area directly under it to high light - probably too high light to easily manage the tank and avoid lots of algae problems. For that reason alone, raising the lights 4-6 inches would be a good idea. And, that would give you better light at the ends of the tank with 4 foot bulbs too.
An 18 inch wide tank can be lighted with only two T5HO bulbs, if they are spaced at least 6 inches apart. One light fixture that might work is http://www.catalinaaquarium.com/stor...oducts_id=1842
You can also get Catalina to make you a custom light using a wider housing to separate the bulbs farther apart. They have made a few that separate the bulbs about 9 inches, which would be perfect for that tank, giving you about 55 to 65 micromols of PAR with the light 24 inches from the substrate. That is a lot of light, and it would mean pressurized CO2 is essential.