4AGE life span
Any engine will last longer if it is operated within reasonable parameters and fed clean oil and air, and routine maintenance. Not considering the weekend replacement items such as seals, timing belt components/cooling system components, the basic block and head should stay mated until some kind of failure. On the 4AGE I have noticed a tendency for the head gasket to fail due to a cupped cylinder head surface, often at the mileage of appx. 170 K.
My first 4AGE blew a HG at this time and I know it was the first time the motor was apart. Bores were well within spec, a simple honing and bearing and seal replacement set that engine back into flawless service after a fly cutting of the head and complete valve job. IMHO, the first thing major that will happen is the BHG. In this case, the crank was well within limits on main and rod journals as well, and this engine had been operated mostly on synthetic Mobil 1 10W30 or 15W50.