I honestly can't seem to find the market segment that the 4 series is after that the 3 and 5 didn't already fill. Other than door counts, these cars already overlap pretty heavily (IMO).
1M: Smallest performance coupe M3: Midsize performance coupe M5: Full size performance sedan M6: Full size performance coupe 7 Series: Oversized sedan
So, ELI5, please.

