Unfortunately, season 5 for Overlord is yet to be announced. However, we should be expecting a new Overlord movie once the season 4 of the anime ends.
The official timeline for the anime is that, first, season 4 will release then it will be followed by one movie. If ever the studio decides to make a 5th season, it’s probably going to be the last season since the source material (light novel) is almost exhausted.
Most fans speculate that the movie after season 4 will feature the Holy Kingdom Arc that is Volumes 12 and 13 in the novels.
As stated in the Overlord Maruyama Fandom, the Holy Kingdom Arc or “Paladins of the Holy Kingdom” is a volume in the novels that will involve a neighbouring country called “The Roble Holy Kingdom”, the premise is that this country typically face regular invasions from demi-humans.
In the events of the Holy Kingdom Arc, a new assault from the demi-humans is happening in the country, this time, they are led a demon emperor named “Jaldabaoth”. The Holy Kingdom then requests aid from the Sorcerer Kingdom, which is pretty ironic because the Sorcerer Kingdom is a Kingdom of the Undead and that the undeads sword enemy of the Holy Kingdom.