second class stamp is 30 p.
You won't need 4 stamps. There is a price calculator here:
http://sg.royalmail.com/portal/rm/PriceFinder? catId=23500532&gear=pricingcalc
For A4 it will be classed as a large letter.
The other factor is the weight. If it's under 100g then the cost seems to be 47p for second class or 61p for first class. If it's over 100g then it costs a bit more.



