I have starting with PS 4 upgraded to PS5, PS5.5 and PS6.
Now I have to reinstall PS due to a system crash. Can I delete PS 4, PS 5 and PS 5.5 when PS 6 is running?
You don't need to install the previous versions.
When you install CS6 (which I assume is an upgrade), it will ask for the serial number for a "qualifying product", just after you have entered the CS6 serial number.
This qualifying product is CS5.5, which you upgraded from. So you put in that, and that's the end of it. It doesn't matter if CS5.5 was in itself an upgrade, it only goes one version back.
If, OTOH, the earlier version is already installed it will verify automatically without asking.