RAT 4.5 does work the Maya 4.5 but the version of MTOR (the plug-in to bridge the gap between Maya and PRman) which comes with RAT 4.5 isn't compatible with it, you can purchace the upgrade of MTOR so that you can use RAT 4.5 again

The upgrade costs $450, which is a lot cheaper than getting RAT 5.5 which is $8,000