And also t5 run really hot believe it or not.
I think Cree LED 18w PAR38 spots would work well for you.
I flowered under them and had very good results (even though I only had 22w/sq ft). I think this would be better because you want the light to be some distance from the plants and not interfere with the main light. (T5HO is large panels that have to be kept close to the canopy. These spots can be kept 18-20" from the canopy. If you buy one, you can shine it against a wall, move closer/further and see how the light focuses. Best coverage, uniformity and intensity is around 18-20" for a spot. 8-10" for a flood.).
Under efficient LED (not epi-whatever diodes like Chinese import lights and their rebranded imposters sold at high prices) you need about 35w/sq ft for tall plants. That's 560w for a 4x4 tent.
The SS 400 is 320w actual in bloom. It's said to be sized for a 3x3' space, which is 36w/sq ft. So you need 240w more That's 13-14 Creel 18w PAR38 spots. If distributed around the perimeter of the tent, that's 3 on each side for 12 total. Maybe go 4 on each side if you want to get closer to 40w/sq ft (possibly past the point of diminishing return, but still more yield).
The way I would mount them is using 1x4" board and an adjustable socket like attached. Use a small length of 1x2 wood on each end to hang from (giving you some leverage to hold the angle of the board against any imbalance the bulbs create). If you attach the sockets correctly, you'll be able to twist them side to side for more flexible aiming. (Might need to use a smooth-face locknut instead of that conduit locknut it comes with. Some Locktight on the threads.).
That's not going to give you completely even coverage. I think you need 4-5 bulbs to cover 4'. You could use more bulbs and a household wall-mount dimmer for LEDs ($25) to reduce the power. That doubles the cost. You could always start with 3 per side (total 12) and add more bulbs and a dimmer later in case you wanted to use it more like a fixture.
That's $288 for bulbs, $120 for sockets, wire, wood. A bit expensive for a crude fixture. But, the bulbs have 10-year warranties, they're a commodity (you can easily add to and replace by just going to the local Home Depot), and they're interchangeable. You could later
mount them to tent legs, fill your whole tent with them. Use them to fill in when a "real" LED fixture fails, etc. They're a long-term investment and not bad at $1.25 per watt for relatively efficient LED with such a long warranty.
If you were doing side-lighting I'd recommend the floods which can be kept closer. But, since you're adding top light around the 1' perimeter of the tent, having them higher might be better. But, closer and a 45-degree angle might not be bad.
