In which of the following pairs, both the species are not isostructural?

Both diamond and silicon carbide have tetrahedral arrangement and central atom is $s{p}^3$ hybridised. Hence, both are isostructural. 

${\mathrm{NH}}_4^{+}$ and ${\mathrm{BF}}_4^{-}$, both are tetrahedral and central atom in both cases is  ${\mathrm{sp}}^3$ hybridised. Hence, both are isostructural. 

${\mathrm{SiCl}}_4 \mathrm{and} {\mathrm{PCl}}_4^{+}$, both are tetrahedral and central atom in both cases is  ${\mathrm{sp}}^3$  hybridised. Hence, both are isostructural. 

In $Xe{F}_{4,}$Xe is ${\mathrm{sp}}^3{\mathrm{d}}^2$ hybridised and structure is square planar while in $Xe{O}_4$, Xe is $s{p}^3$ hybridised and structure is tetrahedral. Hence, both are not isostructural.