Here's why:

* Sodium (Na) is a Group 1 element, forming +1 cations (Na+).

* Silicon (Si) is a Group 14 element, typically forming +4 cations (Si4+) in ionic compounds.

To achieve a neutral compound, the charges must balance. A +4 charge from silicon would require 4 +1 charges from sodium. Therefore, the correct formula for the compound is Na4Si.

However, this compound is unstable and does not exist in a simple ionic form. Silicon and sodium can react to form more complex compounds, such as sodium silicide (NaSi), which is an intermetallic compound with a more complex bonding structure.