I have these for all four of our thermapens, they work great and keep them safe and secure, we are a very busy steak house that temps everything that goes out, the magnet holds it in place and the cover protects well, though removal and cleaning is still required. It will work on the Mk4 but it's a slight stretch, we have it on ours.
I have mine wrapped around a backlit Classic Thermapen. It just works. It keeps the Thermapen handy, stuck to the side of the refrigerator in the house or the legs or shelf of the backyard gas grill. The ability to "stick and forget" the Thermapen while I attend to the food is a real convenience.
The magnetic grip is very firm and secure on any steel surface. Thermoworks did not skimp on the quality of the magnets. The glow-in-the-dark feature glows more than you would expect. I found it surprisingly helpful when using the outdoor grill in the evening.
The thick silicone material is rubbery and grippy. Mine has stayed clean and fresh for the last two years, but my Thermapen gets only occasional household use. I can imagine it getting grimy in a heavy-use professional kitchen, and I wonder whether the grippy texture makes it harder to clean. I admit, I have never found a need to give it more than a cursory cleaning.
I hope this helps someone.