Our back four drops deeper every time one ball goes over us

One ball in behind should change your awareness, not your entire defensive height. After getting caught, centre-backs naturally want an extra few yards, but if the whole line keeps dropping you give the opposition time to play the next pass. Reset by looking at the pressure on the ball. If the passer has time, be ready to protect the space behind. If your midfield is applying real pressure and the passer cannot lift their head, that is usually the moment to recover your normal line. Talk about it as a unit rather than each defender reacting individually. A simple call such as 'hold', 'step' or 'drop' needs everybody to act together. The lesson from the first ball is not necessarily 'defend deeper'. It may be that the line was late to react, the pressure on the ball was poor, or the starting distances were wrong.