Class 2 must be paid by self employed people unless their income is minimal.
Class 2 National Insurance contributions give entitlement to a range of contributory benefits
* Incapacity Benefit
* Maternity Allowance
* Basic State Pension
* Bereavement Benefits
o Widowed Parent's Allowance
o Bereavement Allowance
o Bereavement Payment
Class 4 insurance does not give any more benefits and is payable if your profits are above a limit. It is a stealth tax.
I hope this link works which will explain Class 4:
http://www.hmrc.gov.uk/nic/class4.htm