4" fletch with 125s will work well but you'll be lucky to get them out to anything over 60 yards with a reference level with the target (although I could be wrong, you don't say what poundage you're shooting; I shoot with 55# on my fingers).
I have a set with 125s and a 3" fletch and they fly really well, however... even with my best bow I can only put the rubber band about 1" above the grip at 80.
100 grain piles and a 3" fletch is a good all purpose design, a 2.5" fletch will do on these outdoors and is less affected by wind (just shot a 525 for the Americanround at the Brass Monkey shoot run by Pilgrim Bowen of Boston with a set with 100s and a 2.5" fletch)
I'm thinking of 80s and a 2.5" fletch for Yorks this summer
As for rule of thumb... I would say shoot the biggest heaviest featheriest arrow you can get away with!