The complex ion $[\mathrm{Zn}(\mathrm{NH}{}_3){}_4]{}^{2+}$ is best described as:

$\mathrm{Zn}{}^{2+}$ is $3d^{10}$ with all $3d$ orbitals full, so bonding uses one $4s$ and three $4p$ orbitals. This gives $s{p}^3$ hybridisation and a tetrahedral shape. There are no unpaired electrons, so the ion is diamagnetic.