Price-time priority
Price-time priority is the rule most exchanges use to match orders. The best price fills first; at the same price, the order that arrived first fills first.
If four limit buys wait at the same best bid, a market sell fills them strictly in the order they joined. Moving your order, even by one price step, sends it to the back of the queue at its new price.
Learn more: the order book: how a price is made.