With the Google search, I just recently knew that Chrysler discontinued the ATF+3 oil. That means, dealers started putting ATF+4 into vans originally designed with ATF+3 from certain point.
Is it OK to mix them? I assume the answer should be OK. Otherwise, Chrysler would have caused so much trouble by mixing them. The 1999 T&C I have is at 105k now. The tranny oil was changed at 30k and 60k until now. Maybe by the time at 60k, the oil I got from dealer is still ATF+3.
If there is a problem mixing it, I will try to find whatever compatible ATF+3 oil to do my soon coming trans oil change...