[2021] Are PS4 players able to play with those on the PS5?

So last week, over 5 days, I was trying to matchmake for players to run Jupiter time trial. Waited for 30mins everyday but could not matchmake for anyone.

Two possibilities:

1. The player base on PS4 is dead.

2. Most players have migrated over to PS5 and PS4 players are unable to link up with them.

So, are PS4 and PS5 players able to play with each other? Anyone have any insights on this? There were no mention of this by Massive/Ubisoft, if I'm not wrong.