To determine if species are isostructural, we calculate their steric numbers and hybridization based on VSEPR theory:

• $\text{BCl}_3$ and $\text{BrCl}_3$: $\text{BCl}_3$ has 3 bond pairs and 0 lone pairs ($sp^2$ hybridization, trigonal planar). $\text{BrCl}_3$ has 3 bond pairs and 2 lone pairs ($sp^3d$ hybridization, T-shaped). They are not isostructural.
• $\text{NH}_3$ and $\text{NO}_3^-$: $\text{NH}_3$ has 3 bond pairs and 1 lone pair ($sp^3$ hybridization, trigonal pyramidal). $\text{NO}_3^-$ has 3 bond domains and 0 lone pairs ($sp^2$ hybridization, trigonal planar). They are not isostructural.
• $\text{NF}_3$ and $\text{BF}_3$: $\text{NF}_3$ is $sp^3$ hybridized (trigonal pyramidal), whereas $\text{BF}_3$ is $sp^2$ hybridized (trigonal planar). They are not isostructural.
• $\text{BF}_4^-$ and $\text{NH}_4^+$: In $\text{BF}_4^-$, the central boron atom has 4 valence electrons (3 + 1 from the negative charge) forming 4 bond pairs and 0 lone pairs ($sp^3$ hybridized, tetrahedral). In $\text{NH}_4^+$, the central nitrogen atom has 4 valence electrons (5 - 1 for the positive charge) forming 4 bond pairs and 0 lone pairs ($sp^3$ hybridized, tetrahedral). Since both have the same $sp^3$ hybridization and tetrahedral shape, they are isostructural.