Hi! I accidentally left my DJI Phantom 4  battery on charge for about 4 hours.Can this in any way damage the battery?
Did I overcharge it?

Hi. I appreciate your time bringing this query to us. Could you please try to test it so we could check and determine the issue. Thank you.

It should have shut itself off after being fully charged.

"The Intelligent Flight Battery is designed to stop charging when it is full. However it is a good practice to monitor the charging progress and disconnect the batteries when fully charged."  Source

I've done this a number of times myself, but haven't observed any damage or degradation.

I also believed the software/hardware in the battery should deal with this, although as stated I would not want to leave it unattended or on charge for that long.
The other option if you are concerned is to place the charger on a mains timer, if the battery normally takes say 40-60 mins to fully charge, set the timer to turn off after 1.5 hrs etc.