Heat butter, hot sauce, ketchup, and soy sauce in a small saucepan on grill, stirring occasionally, until butter is melted, about 2 minutes. Brush cauliflower all over with sauce and grill, covered, 10 minutes. Turn cauliflower over, brush with sauce, and grill, covered, 10 minutes. Continue to grill, brushing and turning every 10 minutes and reheating sauce as needed, until cauliflower is lightly charred on all sides and fork-tender, 25–30 minutes. The sauce should be used up by now, but if not, brush any remaining sauce over. Transfer cauliflower to a plate and let cool slightly.