I believe it was the 504 that shared the same capsule as the 421, and that the 604 was different. On toms you won't notice a very bug difference, but on stuff like brass and other tracks the differences will become more apparent.
i tried using a 604 once for trombone and trumpet expecting to achieve the same sound or close to a 421. Didnt even sound close. The 604 was pretty boxy sounding and the high end wasnt as flattering. It definately wasnt as edgy though.
Could work on a few things but definately a different sound than the 421.
I've been using the 604 as a snare mic and have been more than happy with it. As the response curves show, it is rather similar to the 421, but has the advantages of being much smaller and lighter weight, and also gives you the option of using the clip if necessary for live applications.
I usually use the e609 and the MD421 to mic my guitar amp. They definitely sound different. The e609 seems to emphasize the mids/hi-mids more than the MD421. I would describe the MD421 as more balanced and smooth-- maybe even call it a "fuller" sound because it seems to have more low-end response than the e609.