I have two of the two tuner HDHomeRuns. The older units from 3 years ago.
Is there any benefit to replacing this out with one quad unit?
Space maybe. I have a Connect and and Duo, that I use in separate rooms, works great.
It avoids having a splitter in front of the 2 HDHRs which attenuates the signal a bit. Otherwise, I'm not sure there is a benefit.
No need to replace just add the Quad for six tuners less conflicts.
The ones you have are probably fine. And you've already got redundancy built in, if one fails. But if you are considering buying a 4-tuner, you might as well get the new ATSC 3.0 version. And if your antenna is already split (or if you have 2 antennas) it wouldn't hurt to keep one of your 2-tuner units up and running so you have extra tuners.
I'm not concerned about redudancy.
Is the ATSC 3.0 backwards compatible?
Yes