In practice, no. Only the oldest games work on that firmware. And backports for newer games stop at FW 4.xx, as you correctly note.
The answer here is also no. Kits haven't been created for a firmware that low, in part because that firmware is missing so many developer features exposed in newer firmwares. Despite being an evolution of the PS4 OS, it's a bit raw in some respects.
At a minimum, you would need to go to FW 4.00 to play games (FW 4.03 being one of the "golden" firmwares). What you gain is the ability to play backported games, and the ability to install a M.2 SSD when you eventually fill up your PS5 with games. What you lose is maybe the ability to dump keys in the future, but that is such a wishy-washy prospect right now that there is no telling when that may come or if it would let you (as an end-user) do anything useful.
But, if all you want to do is play games, then I personally think you should aim even higher. 7.61 is quite nice. It gets you a ton of quality-of-life features from newer firmwares, compatibility with more games that haven't been backported, and access to Relapse, which is proving to be an exceptionally stable kernel exploit when used in conjunction with YouTube as the userland exploit.
This does violate the "never update" mantra. But right now you're not losing anything by updating to it from 4.03. Maybe someone will come up with something cool and useful just for low firmwares in the future? Then again, we said the same thing about the PS4, and that has yet to happen even after 13 years.