I would be changing the fluid and filter on the 4hp22 every 15k miles. How does it do on the 3/4 shift? Mine has a similar issue that I attribute to a faulty trans computer: it pendulum shifts between 3 and 4 during some slow accelerations (around town driving) when warm. Sometimes the 3/4 shift is abrupt (again, when warm..it's somewhat normal for it to be abrupt when cold), but seeing as it's a ZF reman with less than 1500 miles on it, we're suspecting the computer.
best, whit