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.