Why 4s lose electron to 3d?
In any element after Argon [1s22s22p63s23p6],
the repulsion force between the elctrons increase at a great extent and also the atom becomes very stable. So the elctrons tend to move in far 4s orbital first and after filling it completely , fill the 3d orbital. About chromium and copper, they are exceptions. Any half filled or fully-filled orbitals are more stable then rest of the configurations, so in Chromium one electron of 4s comes to 3d orbital and a stable 3d5 is obtained, similarly in Copper one electron from 4s comes to 3d orbital and forms stable 3d10.

I hope this answers your question, 
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