GT4 settings provide a good base for GT5 settings, especially if the GT4 setting was a good setting to begin with. Most of the time, you'll be able to use the GT4 setting without any tweaks and you'll get an improvement over default, but of course there is room for improvement on top of that.
Also do take into consideration that damper bound and rebound in GT4 have swapped places in GT5. Also GT5 doesn't have as much downforce on road cars like GT4 used to give you.