Which of the following halide is least stable and has doubtful existence:

As we go down the group the stability of halides will decrease due to increase in size of halogen atom, because of this reason the halogens easily disassociates from the compound, so lead iodide is least stable among the halides of 14^th group elements.

 Non existence of PbI_4​ can be explained on the basis of strong oxidising nature of Pb⁺⁴ . 
Pb⁴⁺ + 2I⁻ → Pb²⁺ + I₂​