Hi,
I wanted to play Serious Sam 4 on my Steam Deck today but unfortunaly, black screen.
I tried to reload the local files but nothing change.
Serious Sam Siberian Mayhem working.
Have you got the same bug?

I try to turn it off but the game block :/
I reinstalled Serious Sam 4 yesterday on my Steam Deck but the game doesn't work again.

I upload a video with the problem on Twitter.

https://twitter.com/mental4/status/1663795214870249472?s=46&t=gnIK_KRDC-B6IJrlUCPlpg

I found how to solve the bug.
I switched the Steam Deck to desktop mode and launched Serious Sam 4. The game launched.
I switched the Steam Deck to game mode and launched Serious Sam 4. The game launched ^^

Please try to turn it off and back on again. That helped here. :/