1) Virtually no PC games utilized the OPL4 at all.
2) Those that did still underused the chip like the OPL3 games did to accommodate backwards compatibility. So just like with the OPL3, to hear what the chip was really capable of, you had to look to the chip tune community.
3) However, since the PCM capabilities of this chip are so significant (and they truly are), most of the time the incredible FM capabilities are lost in the shuffle of chip tune artists trying to push this chip hard with lots of PCM. So it ends up becoming in practice, like the later Neo Geo games, a PCM sound system with a splash of FM, rather than an FM sound system - at least in practice.
The ONLY game I could find a video for that utilizes the OPL4 is Duke Nukem 3D. And even then, there are only two tracks posted on YouTube using the chip. One makes fairly decent use of FM channels, while the other one is overwhelmingly sampled. For comparison's sake, I'll also post the OPL3 version of these tracks, as well as the MIDI version.
Here's our first track - Stalker (the OPL4 version)
And finally, the OPL3 (keep in mind, though, the OPL3 IS still being underutilized)
While you can definitely hear the similarities between the OPL4 and OPL3 versions of Stalker, the OPL4 still sounds closer to the MIDI version than the OPL3. And this is MORE FM intensive of the two OPL4 tracks. With the next track, "Grabbag", you will hear almost no affinity between OPL4 and OPL3, but you will hear a lot between OPL4 and MIDI.
Here's the OPL3
And finally, here's the MIDI version
I wish there was more I could show you from this chip from actual games, but I wasn't really able to find anything else. Out of OPL4, OPL3, and MIDI, in general, I did prefer the OPL4, though. It was best of both as it brought in the diversity of samples, but retained some of the character of FM synth.
Still, just like with OPL3, in order to hear what this chip could REALLY do, you have to turn to the chip tunes community. So I'll leave you will a few tracks from various artists. It's interesting to note that many of these chip artists are using the MSX OPL4 chip. That's not amazing in itself, I guess. What's amazing is that the early 80's MSX computer had an OPL4 expansion option to in the first place.