The sonic difference between the MK 4 and the MK 5 in its cardioid setting is slight but potentially audible on direct comparison with a good playback system. The omni setting of the MK 5 has characteristics somewhat like those of the MK 2 S, which is probably Schoeps' most generally useful omni capsule, although again on careful listening a difference would be audible.
Schoeps recommends the MK 5 for any new users if they're not sure which other capsule(s) they want to start out with, and I tend to agree. There's no sacrifice of reliability or performance from its having the two pattern settings, and the characteristics of both patterns are well chosen for a very wide range of recording applications. The mechanically switchable capsule design allows the omni to be a true pressure transducer (full low frequency response, very low sensitivity to wind or vibration, zero proximity effect), while the cardioid is still a true cardioid at low frequencies, unlike the cardioid setting of electrically switched multi-pattern mikes.
--best regards