I have cs4 design premium installed on my machine I want to install CS6 design and web premium trial on the same machine will it mess up my cs4 install?
No it won't but you may face some other changes like file association would be for CS6 application and stuff like that.
Also keep in mind that if you don't plan to go with CS6 and stick with the CS4 then please change the file compatibilty.
Ex: INDD file once got saved in the CS6 cannot be opened in CS4 unless it saved in IDML.
Same goes for the Ai files
hence be precautious of these things.
No, CS6 won't do anything to your CS4. However, be aware that there may be compatibiliuty issues. E.g. if the install replaces a globally used PDF library and CS4 can't handle it, exporting to PDF may not be possible in CS4 programs. Similarly, all file associations will point to CS6 after the install, meaning double-clicking a file will open it in CS6, not Cs4 and things liek invoking Bridge from one of the programs will also use the CS6 version. finally, you may experience issues when removing CS6 and the uninstaller removes a critical global component used by CS4 because it only thinks of its own stuff. So in summary: Within the limitations it will be safe and even if you let the trial run out without making a purchase, it would be best to leave CS6 on teh machine after that to avoid complications.