I had gastric byp 4 days ago. How long before my BMs are normal again?

They will never be "normal" again. You absorb food differently now so your BM's will never be like they were pior to surgery. After a couple months you will settle down into a nice routine of about 2 per day, some people have 1, some people 3. Everyone is different, there is no standard. If you are having constant, runny BM's that are painful, that is DUMPING SYNDROME caused by eating incorrectly.