The 25 is common, and the 21 is rare 
The only 21 I've seen is on ebay just now, although it says that the label has been overwritten 29, which AFAIK doesn't exist. The seller has removed the picture of the underside, so I've attached it below.
Pictures of the 25 are easy to find.
The obvious difference is the spindle drive. On this paltry evidence, the 21 has a commodity brushed motor rather than the Hall-effect drive of the 25.
However, I have an Aiwa with a 4/25 (I'm pretty sure), with a brushed motor
The only 21 I've seen is on ebay just now, although it says that the label has been overwritten 29, which AFAIK doesn't exist. The seller has removed the picture of the underside, so I've attached it below.
Pictures of the 25 are easy to find.
The obvious difference is the spindle drive. On this paltry evidence, the 21 has a commodity brushed motor rather than the Hall-effect drive of the 25.
However, I have an Aiwa with a 4/25 (I'm pretty sure), with a brushed motor
Attachments
This exquisitely ambiguous and ludicrously expensive offering says yes, and no, maybe, in two languages.
JusTone Lasereinheiten. CDM4/11 Ersatz Lasereinheit Laufwerk Lasereinheit, Laserpickup, Laserunit, Pickup, Laser, Laserkopf
The 21 and 25 are both listed in Philips manuals as replacement units for various CDM2 mechs, and appear to be compatible with the same servo board. However, there are several variations of radial motor circuit, and these are not necessarily reflected in the model numbers.
The laser itself is probably compatible, as long as you use the correct procedure for installing a new one. This appears to be the same for all models, and involves pre-adjustment before switching on, so make sure you follow a manual.
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I've seen quite a few of the rarer CDM-4 variants, there's quite a lot of them. I don't have a list of the exact differences between each type, but these are the things that differ between them:
- Motor type, brushed or brushless
- Chassis type, miniature or table, different leg lengths
- Spindle type, clamped, magnetic, ball grip, spring grip, inverted
- RAFOC type, onboard or offboard laser power adjustment
With all those variations you can see why there are so many type numbers. The 4/11, 4/19 and 4/36 are common, but most of the rest are fairly rare, either being made as stopgap replacements for CDM2s or semi-custom items for low volume manufacturers like B&O.
You can generally swap the RAFOC (the swingarm with the laser head) between the different types, just keep a watch out for the rare types that had onboard laser adjustment potentiometers (like most Sony lasers). They were mainly fitted to jukeboxes, and have a potentiometer on the flex foil, like the picture attached. They aren't compatible with other types and vice versa.
- Motor type, brushed or brushless
- Chassis type, miniature or table, different leg lengths
- Spindle type, clamped, magnetic, ball grip, spring grip, inverted
- RAFOC type, onboard or offboard laser power adjustment
With all those variations you can see why there are so many type numbers. The 4/11, 4/19 and 4/36 are common, but most of the rest are fairly rare, either being made as stopgap replacements for CDM2s or semi-custom items for low volume manufacturers like B&O.
You can generally swap the RAFOC (the swingarm with the laser head) between the different types, just keep a watch out for the rare types that had onboard laser adjustment potentiometers (like most Sony lasers). They were mainly fitted to jukeboxes, and have a potentiometer on the flex foil, like the picture attached. They aren't compatible with other types and vice versa.
Attachments
check out post #12 under
https://www.diyaudio.com/community/threads/cdm-4-11-vs-cdm-4-19.84900/
maybe the only differences, which I want to know, are to find in the various spindle motors resp. platter/motor pulley for the compact disc.