The main difference is the Ethernet connection
E4: 100 MBit
M4: 1000 MBit (1 GBit)

This means that if, for example, you set up your WLAN with an E4 and the E4 is connected to your modem via a network cable, your Internet connection is also limited to 100 MBit.

I don't remember what the difference was, but I ended up buying the M4 and it works just fine. Even for a three-storey house with very thick walls, I only need two repeaters.