Wires are measured in cross-section. 6AWG is matched to the 16mm2 wire. The 4mm is not something you would usually look at. This is a pretty heavy wire. And AFAIK there is no standard PV wire with that size. The common options are 4mm2, 6mm2, or 10mm2 (the last one is a bit exotic).
Check the price difference between 6mm2 and 10mm2 and if it justifies roughly 0.5% more power production - go for it.
And to be more on the same side - there is no perfect match between mm2 and AWG wires, but the commonly accepted one for PV wires is:
6 AWG = 16mm2 ~ 0.4% losses (exotic, I've not seen such being offered around, only at remote online stores)
8 AWG = 10mm2 ~ 0.6% losses (again exotic, but can be found around)
10 AWG = 6mm2 ~1% losses (common)
12 AWG = 4mm2 ~1.5% losses (common)
These are the average losses, not the highest ones. But these are what you should be looking at.