Maybe I misunderstood your question,...
but the 424mkIII is an analog/cassette based 4-track Portastudio. It's a great unit, and is worth getting, but it's not digital, it's cassette.
There are many competing machines out there, both new and used, and the choices are many, but I don't know much about the FD-4. I think the FD-4 is a 16-bit/44.1khz machine.
The lowest-end digital Tascam you can get new is the 788, which is a 24-bit/96khz hard disc Portastudio. There's the Tascam 564 "Minidisc Portastudio", which records digital with ATRAC data compression, and is only available used. Also, there's the newly introduced, almost released Tascam Pocketstudio5, which is digital, and to the best of my knowledge it's an audio-to-mp3 recorder, but IMO it's no comparison to the 424mkIII.
Also, in all-digital 4-tracks, there's the newly introduced
BOSS BR532, and the KORG PXR4, which both record to Smartmedia memory cards. Editing features are pretty good on these units, but onboard mixing capability is minimal. I've demo'd the BR532 and PXR4, and though they were impressive, and sound quality is excellent, IMO, the Tascam 424mkIII is still the better value, overall.
Currently, in the US, Guitar Center stores are selling the 424mkIII for $329/US, which is a great deal on a great cassette-based machine.
