Basics of Organic Chemistry

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).