Polarizing capacity/polarizing power: The ability of a cation to distort an anion is called the polarization power of that cation.
Fajan's rules of polarization: According to this rule, the more the size of an anion and more easily will it be polarized and hence the compound will be more non-polar.
And Less the size of cation more will be it's polarizing power.
EXPLANATION:
The size of Li+ is the smallest in size among all the given options. So according to Fajan's rule, Li+ will have the strongest polarizing capacity. Hence option 4 is correct.