Carbocation stability depends on hyperconjugation and resonance. More alkyl groups attached to the positive carbon provide greater electron donation, increasing stability. Tertiary carbocations (3°), where the carbon is bonded to three other carbons, are more stable than secondary (2°), which are more stable than primary (1°). Allylic carbocations, where the positive charge is adjacent to a double bond, gain extra stability from resonance delocalization of the charge.

Comparing the structures: I is 1° (least stable), II is 3°, III is allylic 2°, and IV is 2°. The allylic 2° (III) is more stable than a typical 3° (II) due to resonance. Therefore, the order is III > II > IV > I.

Final Answer: II > III > IV > I