I assume that you're interested in finding an ion that has a 4+ charge and the electron configuration
#"X"^(4+): ["Ar"] 4s^0 3d^3#
A very important thing to notice here is tha tyou're dealing with a transition metal.
Why is this important?
Because you know that transition metals have the 4s-orbital higher in energy than the 3d-orbitals.
This implies that when the cation is formed, the first electrons removed from the ion will come from the 4s-orbital, not from the 3d-orbitals.
Now, notice that you have
You can reform the atom by adding electrons to the cation. This will help you figure out what ion you started with.
So, the first electron added to this ion will be placed in a 3d-orbital, since these are lower in energy. This means that you have
#"X"^(3+): ["Ar"] 4s^0 3d^4#
The second electron added will also be placed in the 3d-orbitals
#"X"^(2+): ["Ar"] 4s^0 3d^5#
The third and fourth electrons must occupy the 4s-orbital because you know that this orbital was emptied first when the cation was formed. This means that you have.
#"X"^(+): ["Ar"] 4s^1 3d^4" "#and #" ""X": ["Ar"] 4s^2 3d^4#
The electron configuration of atom
#["Ar"] 4s^2 3d^5#
Argon has 18 electrons surrounding its nucleus, which means that your atom will have
#18 + 2 + 5 = "25 e"""^(-)#
Therefore, the neutral atom
This implies that