I recently bought the SC Live 4 and wanted to know if there is a specific timeframe for how long the device will be supported.
Judging on the SC5000 support (decks are from 2017), at least 10+ years or more…
If anything Denon has proven since 2017 that they have designed their hardware for the long game, not the short gain.
My SC5000 from 2017 still outperforms and out features a CDJ-3000X or the newest CDJ-1500X. “Industry standard” is sales branding not a design language.
Consider this - even when it’s not “supported” any more, it will still carry on working. It doesn’t need constant firmware updates to survive.
Asssuming you’ve unpacked it and turned it on, you can carry on doing that long past the date it’s no longer sold. Long past the date they release the final firmware version.
The lifetime of the unit is as long as it continues to power up and function. It doesn’t become useless when it’s discontinued or no longer receiving updates..
As easy as it is to complain about insufficient communication, introduced bugs or missing features - this truly deserves praise. 
I have had my sc live 4 for around 3 years it’s my primary device to use for my gigs and I typically play every weekend and has been a work horse thus far
For me, it’s mainly about how much longer the device will receive feature updates. But as you’ve already mentioned, devices from 2017 are still supported, and since the Live 4 launched in late 2022, it should continue to receive updates for quite a while.