4mm2 and 6mm2 are the two cable sizes that appear most often in PV string wiring. The right choice depends on current, run length, installation temperature and voltage-drop target.
A larger cable is not automatically better if the run is short; a smaller cable is not automatically cheaper if it creates losses or inspection issues.
4mm2 is common for standard strings
For many rooftop and utility string runs, 4mm2 H1Z2Z2-K cable is the practical default. It is easy to source and often enough for normal current and distance.
6mm2 helps with long runs and higher current
When the one-way cable run is long or module current is higher, 6mm2 can reduce voltage drop and heat. The decision should come from a calculation, not habit.