Collar the Culprit

The toughness gate is the whole conversation, and it's the cleanest expression of a long-standing white design tension: how do you give the color premium removal without letting it dismantle the small, aggressive creatures white itself wants to play? Splitting the difference by a number does it. Anything with toughness 4 or greater dies; the 2/1s and 3/2s and X/3 hatebears stay safe. That makes this a back-end answer by construction, built to point upward at the fatties and the fortified bodies that white aggro can't punch through, while refusing to be a clean trade against the early curve. It's the inverse of the cheap, low-toughness-targeting removal red and white usually trade in: instead of pinging the small and bouncing off the big, it ignores the small and reliably erases the big. The cost is the price you pay for unconditional destruction at instant speed with no exile clause, no targeting restriction beyond size, and no downside on the stack. Four mana to kill the thing your deck is least equipped to handle is a fair rate when the gate lines up, and dead weight when the board is full of one- and two-toughness creatures it legally cannot touch. The design lives entirely in that conditional, and the condition tells you exactly which deck it was drawn for.