I've used 6" and it works well. With ambient temps at 80 degrees you probably could get away with 4".

I know ARS Cages (my racks) sells their standard 7030 racks with 4" flexwatt. However, I upgrade to 6" for all but the bottom shelf, which I upgrade to 12". I think Freedom Breeder racks most popular heating option is 5", which I think is 4" flexwatt inside of an inch larger encasing.