I don't think they are as different as people think in some ways, which contributes to much of what I dislike about the 3 and 4 Series of this generation.
The M235i (and indeed 2 Series in general) is a great looking nicely equipped car thats fun to drive and has a fantastic engine. The interior is also pretty good for what it is.
The 3 and 4 Series have largely the same interior (the dash is very, very similar) and it doesn't really feel all that much different inside. The 3/4 Series feel like a larger 1/2 Series. I don't think this is acceptable on a car of that type. If you think about the really good 3 Series models BMW have done (G20, E46) the thing they have in common is they felt like smaller bigger cars, not bigger smaller cars.
If you don't do loads of long trips and don't need the fuel efficiency an M235i is a fantastic car. If you do do lots of long trips then do it in something nicer than a 4 Series.
So, really, I'd buy an M235i or something else. I'm not sure which question is truly best answered by a 435d. I've always thought this even before the 4 Series became the most hilariously crap value car on the planet.
Other than offering more space, what exactly does a 4 Series offer over a 2 Series? They both have the same engines, gearbox, largely the same interior, the same options....
When I buy a larger car I'm doing so because I want more than just the smaller car enlarged by 20%. It's because generally I expect a larger car to come with a much nicer interior, more quality, noticeable refinement differences etc etc. The 4 Series doesn't do this, so why bother with it if you also like the M235i?