To answer my own question, I bought an affordable Gen 4 x 4 SSD that falls within the speed threshold of Gen 3's (~3000 Mb/s). Installed, my system is able to fully utilize that speed without detriment and the card appears to be fully compatible.
I have a Dell G15 5515 Ryzen Edition that I'm upgrading the storage on, I'm looking at two different 2Tb M.2 2280 SSDs, virtually everything is the same except one is Gen 3 and the other is Gen 4. The Gen 4 is more durable, costs less, and has similar speeds at ~3500 Mb/s. I'd like to go for the Gen 4, but I'm not sure if my computer supports Gen 4. I was not able to find anything in the user manual or in previous posts, except that exceptionally fast Gen 4's (~7000 Mb/s) may be throttled down to Gen 3 speeds (~4000 Mb/s) by a Gen 3 connector. Seeing as how the Gen 4 I'm considering is within Gen 3 limits, I'm not sure if this will be affected. Any advice is appreciated!
G15 5515 SSD Gen 4 support?