it would have been better if it were 4 inch all the way ,
but you got to do what you got to do,,, thats your
mistake you got to live with.
I doubt the fixture units you have added
will make much difference going into the 3 inch
my only advice might be to put some sort of clean out
at where you have to reduce from the 4 down to the
existing 3 inch cast pipe...
makeing the reduction on a verticle drop into the
3 inch pipe might be wiser . ---instead of on a horizontal
Also reduce down with a 3x4 pvc bell reducer for better flow.
it will most likely work fine as long as you have a
cleanout at the bell near by just in case it were ever to stop
up on occasion
wait and see what happens