[Ni(CN)4]2- is a square planar geometry formed by dsp2 hybridisation and not tetrahedral by sp3. [Ni(CN)4]2- is diamagnetic, so Ni2+ ion has 3d8 outer configuration with two unpaired electrons. For the formation by sp3 hybridisation, the 3d orbital would remain unaffected, consequently, the complex would be paramagnetic like Ni2+ ion itself.
For the formation of square planar structure by dsp2 hybridisation, two unpaired d-electrons are paired up due to energy made available by the approach of ligands, making one of the 3d orbitals empty. By this, there is no unpaired electron and the complex would be diamagnetic.
The dsp2 hybridisation in [Ni(CN)4]2- ion is shown below.