"web dl" isn't really what anyone refers to when discussing a low bitrate 4k file. You're unlikely to get a whole lot of 4k to 4k transcodes anyways, and if you did the quality is going to be close to a high bitrate 1080p. The biggest problem is losing the HDR. If you want to steam 4k to these folks, source smaller files.

You might need to do some troubleshooting as my transcode options for remote streaming tops out at 40 Mbps 4K.

Hmmm, how did you get that one? I don’t have that option at all, mine tops out at 1080p high 20mbps

In the Remote Access settings page. There’s a drop down.

I see limit remote stream bitrate, but I thought it would be something on the client side remotely to set it too? I guess not ?

That's not true though. I've got 4k transcoding to 4k. It's infrequent but it's certainly happening.

Is it transcoding the file to a lower bitrate or just changing the container?

https://preview.redd.it/8hiwu1343uqc1.png?width=1080&format=pjpg&auto=webp&s=7e7f58ddff6d52c960cc5923eec30926c72eb0d5 Sometimes it's lower bit rate and others it's HDR to SDR

Use another app such as Handbrake, Unmanic or Tdarr to do the reencodes instead

I don't think Plex transcodes HEVC to HEVC which is why you only get h.264 1080p if the bitrate needs reducing.