Higher is the hydration energy, more will be the solubility in water. If the cations have different charge then the one with higher charge will have higher hydration energy. Among CsClO4,NaClO4,KClO4 and LiClO4, all of the cations have +1 charge, so for comparing hydration energy size factor will be considered. Smaller is the size more will be the hydration energy. Thus, among all cations Li+ is smallest in size. Hence, LiClO4 is most soluble in water.