Hello Alex,
you are probably aware the huge changes in the eRec application when moving from EhP3 to EhP 4 or any higher enhancement package. The whole user interface for non-candidate roles is completely replaced. This includes the manager as well as the recruiter and the recruiting administrator.
In principle it should be possible to run the old BSP applications as long as you do not activate any business functions for the enhancement packages (aka "pure technical upgrade"). But when I did such a pure technical upgrade from EhP 3 to EhP 4 for a customer about 4 years ago we already faced several serious issues with running the old ui and functions. So what you experience are very likely such issues as well.
Kind Regards
Roman