For first-order ionisation (SN1 pathway) in aqueous ethanol, which order of decreasing rate is correct for (I) $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{Br}$, (II) $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{Cl}$, (III) $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{I}$, (IV) $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{Cl}$?

Both the cation stability and the leaving group matter. Within a given alkyl group, bromide ionises faster than chloride (I > II; III > IV). The change from secondary to tertiary stabilises the cation far more than the change from chloride to iodide helps, so the tertiary halides lead: I > II > III > IV.