Class 4 NIC

[racy tracy]

Hello people, im hoping someone out there will be able to help me. Does anyone know at what point Class 4 National Insurance Contributions are paid and deducted. I'l try and explain........my partner is a self employed/sub contractor builder. His boss pays him a wage every week from which he has already deducted tax......then every 6 months my partner gets an invoice from the HMRC to pay his Class 2 contributions. His last self assessment for April 2011/12 was done by a chartered accountant (which happens to be his boss's sister). She offered to do it for him because my partner has no idea how these things work, and considering she is a fully qualified chartered accountant he didn't think anything wrong with this. However......I feel that her calculations are wrong. His gross earnings for 2011/12 were £23,550, from this she has deducted £908 for expenses leaving an amount of £22,642 then from this she has deducted the personal allowance which then left a figure of £15,463. From this she has then deducted income tax at 20%, (which worked out at about £3,500 approx)........then the next bit is the bit that i dont understand. Going back to the figure of £22,642 she has then deducted Class 4 NI @ 9% of this, which is before any tax was taken off.....which worked out at about £1,600 approx. Is this right?......In other words 1, should he be paying Class 4 and 2, if so should it be deducted from the figure before tax or after?.....someone please help