I'm not a user of Linux Mint, but would say if I want to stay up-to-date on the current gnuradio+gqrx versions on Linux without having to build it all from source, I recommend radioconda:
There's a small learning curve, but that page tells you exactly what to do.
The good news is if you don't like it, you can just remove one folder and most of it will disappear.
You will have to undo one or two things the instructions tell you to do (blacklist, etc) but that's simple.
Otherwise, if you just want to live with the version of gqrx provided by Linux Mint, you can update the rtl-sdr drivers using:
These instructions will give you a driver that supports both V3 and V4.
It could very well be that the driver in Mint already does support V4, the changes have been around for a long time now.