I'm fairly certain that -4's could be either. -5's and -6's have round butt unless I'm mistaken. I just purchased a 686-4(P) seven-shot and it has a round butt - I was told that all of the (P) models have a round butt. Mine has a 4" barrel, and I've had a 686-6(P) with a 4" barrel, both had round butts.
-4's prior to 1996 should be square butt with 1996 being the trasition year. Depending on how soon frames were used up you could possibly find one into late '96; you just never know for sure.