I flower a 4x4 with two 260xls pretty decent. I would definitely agree with a 320 or 350. Its always best imo to have plenty or a little extra than just enough.
You’ll be incredibly happy with the 350R in a 2x4. It’s always a good idea to see if there is a local retailer near you. You can check on HLG’s website. My local grow store beats online prices on fixtures by a good amount. Everyone assumes Amazon or other online giants are cheaper, but that’s not always the case.
The 260 XL and the 320W can be found under Catalog - Kit LED. The preassembled lights are under LED Lamps.
The 260 XL and the 320W share the same heatsink. the 260 has two 288 QBs and the 320 has three - 288 QBs