I still have a couple Mk 4's on two rifles. Considering the current Mk 4 price of 500-600 and you compare the glass quality of a new scope at similar price, the Mk 4 wins-Less CA and better clarity. However, as Kells81 points out, the new scopes have features not found on the Mk 4 such as higher zoom ratio, FFP, etc.
If you want a fixed scope, for the price the MK 4's can hold a good value.