An actual answer (a few actually)
Cassette eight tracks
Tascam made four 8-track cassette units:
238 - Standalone rackmount recorder. Physically, it looks a lot like the Tascam digital multitracks. Records all 8 tracks at once. Ins and outs are -10 dBu on RCA jacks. Has an "Accessory 2" serial port, which allows it to chase sync (and do the full interface with Tascam sync boxes if it's the master). dbx NR switchable in two 4-track banks (and track 8 alone). Sturdily built.
688 - Portastudio (not very "porta" though). Very fully-featured mixer. Ten channel strips which can handle 20 inputs at mixdown. Real meter bridge. Electronic signal routing. MIDI scene changes, etc. Outputs MIDI sync. Records all 8 tracks at once. Has "Accessory 2" serial port, chases sync, etc.
488mk2 - Portastudio. Feature-wise it's more in line with the 424mk2, rather than the 688. Records 4 tracks at once. Two XLR inputs with phantom power (unusual in Portastudios). Used ones are likely to be newer than the other models, as these were still on the market until '98 or '99.
488 - Similar to the 488mk2, but it lacks a sweepable mid EQ and XLR inputs.
Yamaha also made one (I think only one) 8-track cassette unit. I don't know much about it.
Generally they work pretty well, and one that's in good condition is capable of making a nice recording. All of them run at "double speed" (3 3/4 ips). The track format is compatible among all of them, but is not compatible with anything else. They stagger the record/play head on alternate tracks and the head spacing doesn't line up with 4-tracks, so tapes won't happily move between 8-track and 4-track machines.
Syncing 4-tracks
The Tascam 644 has an Accessory 2 port, and will chase sync and interface with Tascam sync units. I don't know if that's the only 4-track with this feature. It may be. Generally, the Tascam units with sync capability sync very well -- typically, they stay within a hundredth of a frame (1/3000ths of a second). If you put the same signal on 2 tracks, one on each machine, there will be a flanging or chorusing effect, but sync is much better than you need to keep different parts in sync.
You could sync two 644s with a Tascam synchronizer (ATS500, e.g.), and they'd work together. Use the master's transport controls, and the slave will follow (rewind, go to locate points, etc.) Or you can make the 644 chase just about anything you record a SMPTE signal to (but you'll have to use both decks 'transport controls to get them close to each other before hitting play).
As noted, you don't get a whole lot out of syncing 4-tracks, though: because you give up a track on each to record SMPTE, you wind up with only 6 tracks. Syncing up two 8-tracks to get 14 tracks works well, though. You can even sync a cassette 8-track and a reel-to-reel, which gives you a lot of flexibility for different uses.