I work a 4 day split and it is going great for me.
I do Chest and Bis on Monday. Shoulders and traps on Weds. Back and Tris on Friday. Legs and Core/Abs on Sunday.
I feel this is best for me and for Weds I just look at it like a shorter lifting day. I feel shoulders should have their own day and so should legs. Mainly because legs is a very intense lifting day and if you're not drained after a good leg workout something is not right.
Also, just to add to the discussion, doing Bis on Chest and Tris on Back day has been amazing. I'm able to accomplish a intensity I never was able to do when doing bis on back day and tris on chest day. Makes sense.