Glad you're on your way to get it sorted out!
A couple of things to note for anyone using the Pi for their cameras. It appears that you can only transcode one stream at a time. Viewing a camera on another Roku TV kicks the first Roku TV out. Also, it appears your Roku TV has to have equal or higher resolution than the stream. You can't watch a 4k stream on 720p TV. It otherwise appears to work well as long as you understand the capabilities. The load on the Pi is extremely minimal.
Using VLC I was able to convert my NVR's feeds from RTSP to MJPEG over HTTP so they could be viewed in IP Camera Viewer.
https://github.com/EnsignRutherford/VLC-Camera-Streams
After installing VLC use these files and modify them appropriately for your RTSP feeds and then point IP Camera Viewer to the new URLS.
Note you should run this on an always-on Windows machine.
This will create one feed that contains all four screens as a mosaic and four individual feeds that you can use.
Nice work!
I also discovered Blue Iris will do the same thing.