K04-S4

Specifically, the code is for a leaking seal buried deep inside the TC and seal is not user replaceable.

Some folks with this code report slightly higher RPM's on the highway and slightly higher trans fluid temps due to TC seal leaking. It is not usually a screaming panic to replace so you should have time to plan and prepare. It's a bit of a job to replace TC so consider installing a good one when the time comes. Among others, 3zero3 is a good place to start for an upgraded TC.

Follow Bentley Manual or similar for Trans Fluid level checks as it must be performed when trans fluid is within a specific warm temp range in order to obtain correct volume. To add ATF fluid, you'll need a ~1 foot length tube with a 100 degree or more bend at the upper end and attached to a fluid pump to push fluid up inside vertical fill tube which is accessible from bottom of trans.