i. Pb has electronic configuration [Xe] 4f14 5d10 6s2 6p2.
ii. Due to poor shielding of 6s2 electrons by inner d and f electrons, it is difficult to remove 6s2 electrons (inert pair).
iii. Thus, due to inert pair effect, +2 oxidation state is more stable than +4 oxidation state.
Hence, PbCl4 is less stable than PbCl2.