I think they are continually balancing or matching what they can sell (to Buyers) with what they can buy (from Sellers) throughout the day .... so they won't do one without the other.
Surely the MMs make the market by adjusting the bid/offer levels to get both sellers and buyers ... if demand is weak they drop the price ... if demand is strong they raise the price ..
My guess is that the 4M is a Sell order at mid-price that the MMs were working all day to fill with Buys .... hence the volume and price depression ..?