Dude, there's at least a 1001 ways to spin the answer to your question.
Rackmounting of the 424 itself is the most difficult aspect of your request. There's no rack kit for it, and if you DIY, you'd have to be extremely careful that the screws that hold the rack rails onto the Porta would not enter the case and short something out inside the Porta.
The other part of the question, the rack gear you'd probably want, is another question to which there's no definite answer.
What I have posted, above, is:
[2] Tascam MX-80, 8x2 rackmount preamps/mixer
[1] Tascam PE-40, 4-channel/4-band fully parametric EQ
[1] M-1B, 8x2 line mixer
[1] MH-40, 4-banger headphone amp
[1] Tascam 234, rackmount 4-track cassette, with built in 4x2 stereo output/cue mixer.
This type of gear presents numerous and almost unlimited patching combinations, situationally dependent on what kind of gig you're recording.
Other gear, not pictured, that I'd recommend, might be:
Fostex 3180, stereo spring reverb
Fostex 3170, stereo compressor/limiter
Fostex 3030, digital delay
Tascam GE-20, stereo 10-band graphic EQ
Tascam PB-32R or PB-64R, RCA patch bays
Or, some comparable rack- or half-rack delay/reverb processor, such as my Yamaha R100, stereo digital delay/reverb, half-rack sized.
Notice, all this gear I've quoted is vintage gear items, only found on Ebay, and other used gear venues, but it represents a heck of a lot of bang for your buck, at the current low-bid prices.
There's also another vintage piece, that's rare indeed, the Tascam GA-40, 4 channel noise gate. I've only seen one of these GA-40's on Ebay in over 3 years.
There's more, I'm sure, depending on who you ask, but that's my answer. If you're talking about NEW rack gear, dude, there's loads of it, which you could spend the rest of the year talking about, wondering about, and/or comparing.
Rack gear is handy and versatile, and is still applicable to driving signal into the likes of a Tascam 424. The rackmount of the 424 is the most difficult part of the proposition.
Do you have the 424std, mk2 or mk3??