Hi. In April, 2015 Schoeps decided to unify the naming of their special/alternate capsules. In the process, the MK 4 S became known as the MK 4 XP. The characteristics weren't changed--only the name.
Around the same time a new, variant, the MK 4 P, was introduced. Like the MK 4 XP, it has the same midrange and high-frequency response as the standard MK 4 plus a low-frequency rolloff--but that rolloff starts lower in the MK 4 P, so that overall, it has less effect and/or would compensate for miking that's not as close. The slope of the rolloff is the same as in the capsules you have--gradual, less than 6 dB/octave, not steep like an electronic "high pass" filter which would typically be 12 or 18 dB/octave.
Either capsule can be suitable for music recording when the source material and/or the recording venue overemphasize the bass. Or they can be used for close-miking an instrument or voice, to counteract proximity effect. However, sonically the result is hardly any different from what you would get by equalizing the output of a standard capsule.
--best regards
The "P" stands for "proximity compensation"; "XP" = stronger compensation for closer miking, i.e. the rolloff begins at a somewhat higher frequency.