I think generally there is the perception that 4 cylinder engines with high pressure turbo charging have problems in high performance applications.
1) Excessive turbo lag in high pressure turbo engines.
2) Typically inadequate cooling and heat-soak issues that go against the concept of a vehicle with //Motorsport badges.
3) Highly stressed components that lack the durability found in a larger displacement 6 cylinder engine. Buyers of prestige brands have very high expectations of durability!
4) Will not sound like a BMW should.
5) 4 cylinder engine will have more vibrations due to imbalanced harmonics.
6) Gives the impression of cost cutting, so you are basically getting less for your money.