MTN Racer is the shoe Topo points at a mountain race. It carries a 33 mm heel, so there is real foam under you for long descents, but it sits lower than the Ultraventure 4's 35 mm and feels quicker for it.
It suits you if you race trail, or run long in the mountains and want a shoe you can push in rather than one you merely survive in.
It is not a beginner's trail shoe. At 295 g on a 5 mm drop it asks your feet and calves to do their share, and there is no rock plate, so the sharpest ground still comes through. If your trail running is mostly easy hours, the Ultraventure 4 is the softer, steadier version of the same idea.