Hi,
I have Master CS3 and I plan get CS4 and install on my computer and I want keep CS3 for my clients.

I wonder will work both? will any problem?

Should...I actually had CS2, CS3 and CS4 Photoshop running on one machine. Mostly my laziness in uninstalling but they all functioned!

I agree. Adobe rarely creates new versions that knowingly torpedo older versions.

I run multiple versions of several Adobe products...to a point of distraction actually...and I can't remember ever having a version interference issue.

We have both CS3 and CS4 Master Collection installed on the same computer without any issues, as we design our Motion Menu templates in After Effects CS3 for compatibility for our customers, and need to test in CS4.

We frequently open up all the programs in both versions without issues.