Hopefully Joel will be along to give you a better answer, but I believe for CS4 you will need either the World Tools plugin from In-Tools.com or a script to enable the World Ready Composer.
The World Tools plugin would be overkill for this one. Properly encoded Tamil fonts should work if you turn the World-Ready Composer on. The script "r2l Assign World-Ready Paragraph Composer (to selection)" doesn't actually assign right-to-left behavior, if I recall correctly; it only turns on the WRC.
The above is true only for "properly-encoded Tamil fonts" so if it doesn't work for you, please do come back and tell us what fonts you're using. I've never run into a Unicode Tamil font that was broken, but there's a first time for everything.