I have had good luck so far with my lights from HLG, I have a 550 R-Spec that’s a good match for a 4x4, in the 3x4 I have a 135w QB panel kit from them for veg, and a homemade build with their emitters that’s 320w for flowering in there. There are cheaper and better options for DIY, but if you wanted to try it, I think they’re the easiest option for a beginner. A 320w light kit with the XL heatsink so the three panels are better spaced out for an elongated space would fit the 3x4 well:
And if you want to read or ask questions about DIY using the popular Bridgelux strips, there’s a very active discussion here: