Plant count doesn’t really decide this, canopy size does.

Two plants can fill a 5x5 if you veg them long enough and give the root zone enough volume. Four plants just get you there faster. If you think you’ll ever want to flower the whole tent, I’d rather have the 5x5 light and dim it than buy a smaller light and end up patching coverage later.

The SE7000 dimmed down will usually be more useful than a 2x4 fixture blasting one strip of the tent. Better spread, softer intensity, less chasing hot spots. In organic soil especially, I don’t like running more light than the root system and biology can keep up with. Too much photon pressure just dries pots faster and pushes the plant harder than the rhizosphere is cycling nutrients.

A 2x4 light makes sense if you’re truly only going to use half the tent most of the time, or you like modular setups. But for a 5x5, I’d buy the larger bar light once, dim it, and shape the canopy to match the footprint. Then use height and dimmer to tune it instead of trying to make the tent fit the light.