Carefull buying ROM's, I haven't checked Amigakit for a while, but last I know they were only selling A4000T rom's. While these do work in your A4000, your standard 3.0 installation will not work with these rom's (it will work fine with regular 4000D 3.1 rom's).
This is because the 4000T rom's are missing workbench.library, which in turn means you need a special set of installation disks which has the lib on disk and installs it to LIBS: on your boot drive.