They're gonna use pressure treated ground contact posts in concrete .... right?
I'd do 6" ... every what ... 8' ... cost difference is not bigly ... divided by two.... meh.
Easy decision ... I've used 4" up until now ... and plan a lot more....real soon...
I'll prolly use 4"
6 is better tho' ... 4 is good enough for me.
HTH .... LOL
I dunno???