Let's say rocks start at 10 and decrement their way to 0 as we mine bits of it.
As it happens now
counter = 2.
M1 starts to mine the rock.
M4 starts to mine the rock.
M4 finishes and collects chunks.
counter = 1.
M4 starts to mine the rock.
M4 finishes and collects chunks.
counter = 0.
M1's mining is cancelled.
Start the spawn timer.
As I'm proposing
counter = 2.
M1 starts to mine the rock.
counter = 1.
M4 starts to mine the rock.
counter = 0.
M4 finishes and collects chunks.
M4 tries to mine the rock again, but the menu choice isn't available.
M1 finishes and collects chunks.
Start the spawn timer.