Silicone pad sounds great- there's more than one good way to do things. It's like in medicine: First, do no harm.

Also: Thermostats are only good when correctly installed. Ask if not sure.
Yes- you just figured it out. Sooner or later, snakes burrow- substrate gets pushed away, & that hot floor underneath can "burn" the snake. Again, no (!) surface temperature is safe for a snake to lay on when it's over 90*. And you're asking for trouble with 3-4" of substrate, because that is actually "insulation"- it's going to prevent most of the heat from rising into the snake's home where it's needed, and might even overheat the UTH. No wonder it came unstuck...
Keep the substrate to no more than 1" deep over UTH- many say .5"- but it depends on the kind of substrate- some is more dense than other kinds. BPs are not big at burrowing either, so it's not like yours will be suffering from the lack of burrowing material- but if you want, you could always give him a deep container-full to burrow in on the "cool" side of his home instead to "play in"- just make sure it's never over the UTH. See? 3-4" of substrate is a waste of resources anyway.
One more thing that comes up now & then- remember you're roughly 98.6*, so that 90* surface won't feel hot to you, it will feel cool. Same goes for water (people sometimes try to soak or wash their snakes in water that feels nice & warm to them, only it feels HOT to their poor pet. So get & use an accurate thermometer ("temp. gun"). Many snakes have been known to sustain "burn" injuries after laying on surfaces that were too warm to be safe for them, & for unknown reasons they don't get off in time- burns are painful, may become infected & ooze, take a long time to heal, & may be fatal depending on the severity. A snake may first show some redness (pinkness) on their ventral scales, so watch for that. (Which is confusing because some snakes also do that pre-shed. You need to be a "detective" when you keep snakes-

)
And btw, you seem to be catching on quite fast-

so don't feel bad for getting some things wrong...feel GREAT for asking & learning better ways to do this stuff.