The geometry and magnetic behaviour of the complex $\left[\mathrm{Ni}\right(\mathrm{CO}{)}_4]$ are 

$\mathrm{Ni}\left(28\right):\left[\mathrm{Ar}\right]3{\mathrm{d}}^{8}4{\mathrm{s}}^2$

$\because$ CO is a strong field ligand, so unpaired electrons get paired. Hence, configuration would

For, four 'CO' ligands hybridisation would be ${\mathrm{sp}}^3$ and thus the complex would be diamagnetic and of tetrahedral geometry.