Will installing PS4 and ACR 5 wreck my current PS3 and ACR 4?
If I can humbly add an extra comment to the DEFINITIVE EXPERT answers;
Installing the upgrade version of CS4 without removing CS3 also leaves CS3 functional. Now I just have to figure out if I can safely remove CS3 without messing up anything.
However this is the paid for (upgrade) version. With an install of a trial version a couple of numbers back, removal of the trial damaged Bridge and after a lot of troubleshooting, I had to completely reinstall the whole earlier suite (I have CS2 suite as well) to get back. Since that was an upgrade as well it ended up being a painful and long winded recovery.
The best solution is just to buy whatever version of CS4 you qualify for and forget about trials. There are a LOT of really useful improvements as well as a big leap with layers and masks and non-modal edits. Brilliant!