I have three Model 4's, this one was used last year by one of my students for his class project.
TRS 80 Model 4 with replacement high capacity 1/2-height disk drive.
I installed a replacement drive with 80 tracks / more capacity. Tested, working. If you look just above the new drive you can still see the old metal plate that was covering the original full-height drive. It looks funny so I removed it.