Question: why pbcl4 does not exist?
Answer: Chlorine is a stronger oxidising agent and so can easily oxidise lead from +2 oxidation state to +4 oxidation state, therefore, PbCl4 exists whereas PbBr4 and Pbl4 do not exist because bromine and iodine are weaker oxidising agents and so cannot oxidise Pb2+ to Pb4+.