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

In $[{\text{Ni(CO)}}_4]$:$\text{Ni}$ is in $0$ oxidation state ($3d^{8}4s^2$).$\text{CO}$ is a strong field ligand, forcing all 10 valence electrons to pair up in the $3d$ subshell ($3d^{10}$).The empty $4s$ and three $4p$ orbitals hybridize to form $s{p}^3$ orbitals.$s{p}^3$ hybridization gives a tetrahedral geometry.All electrons are paired, making it diamagnetic.