Plutonium (Z = 94) has the ground-state configuration $[\mathrm{Rn}]5f^{6}7s^2$. The number of unpaired electrons in $\mathrm{Pu}{}^{4+}$ is

Removing the two $7s$ electrons and two $5f$ electrons gives $\mathrm{Pu}{}^{4+}$ with $5f^4$. Four electrons occupy four separate $5f$ orbitals with parallel spins, so there are 4 unpaired electrons.