Si^4- is larger in size, so the attraction between the nucleus and valence e-s is weaker and its e- cloud is more polarizable, therefore, Si^4- is easier to donate e-s.

But, to compare C^4- & Si^4- stability, C^4- should be more unstable than Si^4- because of its larger electron-electron repulsion.

If we compare the basicity of F- & I-, I- is not quite available to donate e- than F-, because I- is more stable than F-.

So, there seems to have 2 factors to explain this case, which one is dominant?