I have P4, P5, and P6 all installed on the same machine.  No problems to speak of.  The only thing that annoys me sometimes is when I double-click on a .pz3 and it automatically opens, for example, with P6 when my intention was to open it with P5 (just a minor thing).  The advantage is that you can add the P4 runtime to P5.  That way, you don't have to move all of your collected P4 content over to the P5 runtime folders.