Was talking to Stevo about this recently. He trains in a similar method to you, coupling back with biceps and chest with triceps. I work on the principle that once i've completed my chest exercises, my triceps are too fatigued to do anymore. I therefore tend to do chest/biceps and back/triceps.
No I don't (although there's nothing wrong with doing that)
Shoulders/Triceps
Back
Chest/Biceps
LEGS!
Those are my 4 days.
Edit: And I've just gone back to the posts and realised I did say I did that. Which is bizarre because I don't I think maybe I was talking specifically about a 3 day split.
Anyway, I think either way has it's benefits. I'd go with whichever you prefer.