My pi3b can handle 10 bit HEVC files with MMAL disallowed, has trouble with it allowed but tries. No codec license.
My pi4b 1gb handles the file fine no MMAL.
I don't know what paying for the license would do on a pi3 or pi4.
You can't purchase codec licences for the Pi 4B - they are only available for the Pi 3B+ and below. It's assumed that MPEG2 and VC-1 (the two licensable codecs) can be decoded in software on the Pi 4B.
On the Pi 3B+ and below the MPEG2 and VC-1 software licences can reduce the CPU load on content that could otherwise be decoded in software, and for interlaced MPEG2 and VC-1 this can be significant. I'm not saying you need the licences - just that having them will reduce your CPU load (which in some cases may be desirable). On the single core Pis (Original A and B, A+ and B+, Zero, CM1 etc.) the hardware licences are even more important if you want to decode MPEG2 or VC-1 (particularly interlaced HD)