Structure I stabilises by resonance and has 6-α-H atoms for stabilization through hyperconjugation. Structure II has 5-α-H atoms for stability through hyperconjugation. Structure III stabilises by resonance and has 3-α-H atoms for stabilization through hyperconjugation. Structure IV has only 2-α-H atoms for stability through hyperconjugation.
So, the order of stability is (I) > (III) > (II) > (IV).