I can see absolutely no difference in arrow flight between 75/105 or 4x90 out of my bows, both fly perfectly. Haven't tried 60/120 but see no need to, unless I get bored one day and need something else to test. I use 4x90 for the same reason Deadsmple does and honestly, I like the symetry.
I agree with Mark: I do prefer 4 fletch whole heartedly, and am completely convinced in my little mind, however small it might be :rolleyes: , that it's superior to 3 fletch. Espcially when coupled with lower profile feathers.