I went from a 420d to a 640d and back to a 4 series. I agree with the above, the bigger BMW are more refined, but are dead to drive. The 4 series has a lovely interior, but in the non M the leather can be a bit naff. I dont know, if your not going for outright performance then it seems a lateral move at best. 520d or 430d performance will be similar enough. Practicality is down on the 4, the rear seats are super crampped.

If its driver focused get a 435d good value on x drive models around.
https://www.carzone.ie/used-cars/bmw...03541349249814
I know this car and its owner, he went to a m4. Great car.