4s subshell always fills before the 3d subshell except some exceptions.
For Chromium, instead of [Ar]4s2,3d4, the 4s subshell loses an electron to the 3d subshell, so chromium is actually [Ar]4s1,3d5.
The same happens for copper, which is [Ar]4s1,3d10 rather than [Ar]4s2,3d9
When forming ions, electrons are lost from the 4s subshell first. For example the ion Cu+ would be [Ar]4s0,3d10 (you can just not write the 4s if you like) as opposed to [Ar]4s1,3d9