NIM24081 - Class 4 NICs: Structure: Special Class 4 NICs: Calculation of Earnings
Regulation 105, SS(C)R 2001
Earnings for the purposes of special Class 4 NICs are the same as those for the purposes of Class 1 NICs (see NIM02000 onwards).
However, the Class 4 NICs payable are calculated at the same percentage rates as ordinary Class 4 NICs and not the equivalent Class 1 rates, see NIM24200.