How can I attach a Raspi (4) behind the monitor or something?

I don't have a case that has extra holes to clap the part on the wall. Lying on the table would sometimes be inconvenient when the PS4 is still on, because the fans would point exactly at the Rapsi…

As an expensive alternative, I would buy a wall bracket for the console for 30 euro (or hopefully less) and hang it on.

Glue in place with double-sided tape.

Mine has been lying on the table without a rum for 5 years. It doesn't itch.

Are you sure that's strong enough? Because the monitor is not completely "smooth", but rather curved at the back

So that's a bit thicker.

It's on woodchip wallpaper.

The Pi doesn't weigh much either.

Secure with long cable ties at the back of the monitor bracket? That's how I've done it before.

^ Here is a picture.

I put double-sided adhesive tape (1) on the back and, as you can see, also cable ties (2).

1: So that it doesn't slip & 2: For extra hold.