I have recently switched to the 4x4" format and have not noticed any significant loss of speed or any gain/loss of stability. I still have several arrows fletched 3x5" .
I' ll probably stay with 4 fletch configuration given the economics ( 2 4" feathers from a single full length one) and the benefit of not having to ' look for' a dedicated cock feather. Besides, I like the appearance of a 4 fletch.

Mark