Sodium forms a highly positive ion which holds the negative chlorine ions very tightly in an “ionic” bond. SiSl4 is more “covalent”, it the silicon shares the electrons from the chlorine, unlike the sodium which hogs them all for itself. Also important is the shape of the molecules. Salt is linear, it has a positive end and a negative end and in the crystal the ends line up with opposite polarities close together. Like how some stick magnets would line up. The silicon tetrachloride has its positive ion shielded behind 4 chlorine ions. Therefore, the molecule next to it sees mainly negative chlorine which wants to be repelled by the second molecules chlorine ions. This compound will fit together in a crystal more by how easily it can pack together rather than by positioning + and – charges. That’s why the crystals melt at different temps. I’ve never worked with SiCl4, but the principles are basic throughout chemistry. I could give several more reasons also that have to do with molecular