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

To determine if species are isostructural, we need to check their geometry or shape based on VSEPR theory and hybridisation:

• $\text{SiCl}_4$ and $\text{PCl}_4^+$: Silicon in $\text{SiCl}_4$ has 4 valence electrons, forming 4 bond pairs with 0 lone pairs ($sp^3$, tetrahedral). Phosphorus in $\text{PCl}_4^+$ has 5 valence electrons, but a +1 charge leaves 4 electrons; it forms 4 bond pairs with 0 lone pairs ($sp^3$, tetrahedral). They are isostructural.
• Diamond and Silicon carbide (SiC): Both possess a giant three-dimensional network structure where each atom is $sp^3$ hybridised and tetrahedrally bonded to four other atoms. They are isostructural.
• $\text{NH}_3$ and $\text{PH}_3$: Both central atoms belong to Group 15, having 5 valence electrons. They form 3 bond pairs and have 1 lone pair. Both possess a trigonal pyramidal shape. They are isostructural.
• $\text{XeF}_4$ and $\text{XeO}_4$: Xenon in $\text{XeF}_4$ has 8 valence electrons, forming 4 single bonds with fluorine and leaving 2 lone pairs. Its steric number is 6 ($sp^3d^2$), and its shape is square planar. Xenon in $\text{XeO}_4$ forms 4 double bonds with oxygen using all 8 of its valence electrons, leaving 0 lone pairs. Its steric number is 4 ($sp^3$), and its shape is tetrahedral. Therefore, they are not isostructural.