dexron in a 904? help please

I just picked up a 1986 ram royal 150 van. its basically a B-van. going over the fluids and what not just getting to know her, The trans dip stick says use dexron on it. I have always atf+4 in every mopar from old a-body's to my 98 ram truck. I plan to drop the pan and do a filter service should I use dexron or +4?
thank you for any help.