SiF4 and SF4 are not isostructural because SiF4 is tetrahedral due to sp3 hybridisation while SF4 has distorted tetrahedral geometry due to presence of lone pair. 


Related Theory Two molecules are isostructural if they have a similar number of atoms arranged in a similar structure (spatial coordinates). Caution If they have same number of lone pair and bond pair, they are considered as isostructural.