Not 4 or 5, just the Light and Void would be enough.
It would be possible to feature 3 forces. Patch 7.3 of Legion indeed featured 3 forces: the Burning Legion (naturally), the Army of the Light, and the Void in Mac'aree.
A section of the final raid could feature a minion of the Light or the Void trying to stop the Janitor, that would be interesting to see (the creature could either get in our way or try to help us). Or conversely the forces of the Light could come to the Janitor's aid, since it was stated in a whisper that the Light made a bargain with Janitor (the "enemy of all" from the perspective of the Old God N'Zoth).
There is also a precedent of the final patch of an expansion featuring elements that appear in the following expansion (Iron Star in 5.3, Legion in 6.2), so personally it would make sense to feature the Lightbound in some way, since they'll obviously appear in 10.0. Perhaps what the Janitor is trying to do could serve as prelude for an invasion of the Lightbound in MU Azeroth. We also know that the Lightbound are trying to conquer the entire Cosmos, so it makes sense that they would get involved with the Cosmic shenanigans the Janitor is trying to do.
Of course, nothing since patch 8.0 is comparable to patch 7.3, which in my opinion is the best patch in this game's history (at least when it comes to the Lore and Story for certain), but they can try.
Also the next expansion will be Legion 2.0, at least from a hype perspective. So expect the Void Lords to be featured massively. The Void Lords have pretty much been the overarching villains of the franchise since Chronicles came out. As they are the begins responsible for Sargeras' descent into madness and the creation of the Old Gods.
What better time to feature the Void Lords than in the expansion after ShitLands?