My car has back and rear brakes , I am wanting to know how long do the brakes last and how much do they cost to replace. My car is nearly almost 4 yr old.

Car is going in for it's service in November and at the time new cambelt/water pump will be fitted but also thinking of new brakes?.

would that be discs front and rear?

Yes.

well it would depend how much you use them,you could do a visual inspection through the wheel and see how much meat is left on them

How much are you talking for replacements on all corners if I do need to replace them.

not a lot i wouldn't have thought,rears probably won't need doing,maybe £60 for a set on the front(using the logic that mine were £100 for ferrodo ds2500 and standards should be about 40% off maybe)its only a five minute job to fit them too easy diy