I host anywhere from 1-6 streams at a time on a custom built server.. I'd doubt you're going to get what you're asking without a much bigger build. It doesn't take a crazy build for plex but if you ever need transcoding the pi can't do it. My most recent iteration was ~$1300 but the price was mostly storage.

It'll only work if you can direct play everything and depending on your ISP upload speed, you may need to transcode any remote streams in which case a Pi will probably burst into flames. A comparable option would be a micro form factor office PC like an Optiplex with a recent gen Intel processor. The iGPU in these works great for transcoding.

If you want something more serious you can do a custom built PC with a Fractal Design case (Define 5, 6, 7 or Node series) which are made to hold a ton of 3.5" drives. Again here all you really need is a newer Intel CPU and SATA ports (and/or an LSI SAS card).

From my understanding pi doesn't have the greatest hardware for streaming so I wouldn't trust it for multiple devices at 4k.