Typo error: draw only 1 electron for 4s because it is 4s1 and not 4s2. This is an A level Chemistry question and not a secondary biology question.

A half filled or fully filled d subshell is relatively more stable compared to other variants and it has to do with quantum. Instead of 3d4 4s2, you rewrite it as 3d5 4s1. The same principle applies to copper.