What does it mean that an electron quantum numbers are n1= 4 - l1?
$n_1 = 4 - l_1$ means the same as $n_1 + l_1 = 4$, so the $n$ and $l$ quantum numbers of the first electron added up should equal 4. This could refer to 3p or 4s.

For the second electron removed, the $n$ and $l$ add up to 5. This could refer to 3d, 4p or 5s.

For chromium, the first electron removed is the 4s, followed by the 3d. For copper, this is the case as well, but its mass is higher than that of iron, so it is not the answer.