What model does this unit succeed? And what are the differences with the higher grade MEP-7000?

This unit does not succeed any particular model.

The main difference with the MEP-7000 is in compatibility with the “rekordbox” music management software. Also, on the MEP-7000, the control unit has a 3U depth, while on the MEP-4000 it has a 2U depth for a more compact size. For details, see the comparative function table in catalogues, etc.