Okay. The basics.
You have your 424mkIII which records 4 tracks onto cassette. It's a Portastudio, so you can mix, record and playback, but you can't play that tape directly in normal cassette decks. You need to mix your 4 track recording from the 424mkIII to a player-compatible media. I'll be honest w/you, that these days it's considered that CDR is a standard recordable listening media that's universal, but in the old days we used cassettes or R/R analog decks to mix down to. Cassette was pretty universal as a mixdown media for homers.
Take the Tascam 102, it is a standard cassette deck. It's considered your high quality home hifi cassette deck, not a low end boom box or crappy department store cassette player. This unit is compatible with all other consumer format cassette decks, such as other hifi home stereos, boom boxes and your car stereo.
Mix down from the 424mkIII and dub it to a tape on the Tascam 102, then you have a universally listenable and portable medium for your 4-track jams.
Other approaches would have you dub mixes from the 424mkIII to the 'puter soundcard or your standalone CD recorder, but given your choice of equipment & for the sake of this discussion we're talking analog.
Cassette is generally regarded these days as a crap- medium, but it can be a decent performer, is cheap and flexible & has it's small pool of supporters. All 3 units you've mentioned in the opening post I'd consider high quality cassette recorders.
The 124AV is another beast entirely. It's a mastering deck of another sort than the 102, plus a few really unique features. It's a hotrodded cassette format that's not compatible with your 102 or home stereo, boom box or car stereo cassette. Not compatible with commercial standard cassettes, but... it's a halftrack mastering deck... giving twice the tape width of a standard consumer grade cassette.
*The standard consumer cassette uses half the width of the tape in two passes (in two directions), Side A and Side B. The halftrack format uses the entire width of tape in one pass & one direction. More tape width means more headroom,... which is a good thing (better signal-to-noise ratio).
Tho' the 124AV runs single (normal) speed,... (I always thought they'd have the-bomb killer hifi if it ran double speed, but),... it also has a Left/Right overdubbing capability, i.e., you may record the left track separately from the right track in sync. That makes it interesting for the most basic of stereo tracking or minimal overdubbing production. This feature is convenient for music production, but it was originally scoped & designed to accommodate recording "control pulses" of the type that old fashioned slide projectors used to use. (Think: 70's tech).
So the caveat is the halftrack (124AV) format will give you a beefier hifi sound than the standard consumer (102) cassette format, but it's tapes are not compatible with standard players. You're kind of stuck in the same boat as with the 424mkIII, being a specialized format.
Also, the 124AV has a "crossfeed" button that gives a very slight "mix" of the Left and Right tracks on playback, as two separately recorded tracks might audibly benefit from being positioned a little bit toward center, as opposed to L/R being in discrete speakers at the extreme left and right. It's an interesting feature. Bear in mind that this was the first cassette overdubbing product whatsoever, which even predated the 4-track cassette Portastudio itself. It was obviously specialized for Audio-Video applications and fixed installations, but it lends itself well to simple audio production for hardcore tapeheads and hifi enthusiasts.
In short, you have the multitracker (Portastudio), and you either play back tapes all the time off that, or you mix down to another tape & listen to the mix tape. If you want a mix tape that sounds superior that you'd only listen to at home, use the 124AV. If you want something that sounds decent and is compatible everywhere, you can take in your car or give to friends, use the 102 as mixdown medium.
Thanx for asking. Hope that helps!
