If you installed it correctly, the new FFS should have an identifier of DOS\7 (a.k.a. 444F5307) and the older FFS is DOS\3 (a.k.a. 444F5303), so they should not interfere. However, if you use the early startup menu to select a boot partition you need to do this twice because each of the systems reboots once on power-up.
Actually you may skip the OS4 partition selection for a second time, if you add the "MODULE Kickstart/BootDevice" entry in the Kickstart/Kicklayout file, by typing
Echo >Kickstart/BootDevice "DH2"
(assuming DH2 as the OS4 partition).
Once in OS4, you may reset and boot again in OS4 with no ESM selection. To return in 3.x, just keep pressed Ctrl+A+A for a few seconds ;-)
