Depends really what you want the mod(s) to do for you. IMO, the original pickup are a bit thin sounding but the preamp is good and doesn't too much colour the Jazz sound.
For my MM4, I changed my pickups to Dimarzio Ultra J (some said are very similar to the Sadowsky Humbuckers, and to me they are too) and the preamp to John East - Marcus Retro. It is now a Jazz on steriod! with tons of tonal flexibility! My mods completely transformed my MM4 into a different animal IMO, but that was what I was looking for! That "Pull for Bright" knob is DANGEROUS -
. BTW, Mr East is friendly, service is ace and delivery is quick.
Regarding Pickups, Jason Lollar single coil jazz is a good choice to consider, if do not want to deviate from the Jazz sound (One of the TBers put a pair of JLs in his MM4, he should be able to share his experience wth you, do the search on "Lollar" here and PM him) I bought a pair of JLs for my 71' Jazz to replace the Fender CS PU. The JLs are a bit darker and warmer than the Fender CS ones, the bridge PU is particularly wicked when on its own. Again they suit what I was looking for, too. BTW, Lollar PUs service is good and delivery is quick, too.
