The increasing order of radii of $\mathrm{Sn}{}^{4+}$, $\mathrm{Pb}{}^{4+}$, $\mathrm{Sn}{}^{2+}$ and $\mathrm{Pb}{}^{2+}$ is:

The higher charge (+4) removes more electrons (the whole outer $p$ and $s$ pair) and gives much smaller ions than +2 of either element. Within the same charge, Pb is below Sn, so Pb is larger. This gives the order stated.