Could be OLD GAS that you mentioned. 4 year old gas, even mixed with fresh fuel, will gum up the intake valve stems which will make the valves stick, which will cause the push rods to bend and flatten lifters/cam lobes.
Happened to me. I had to pull the heads and hammer out the valves and use lacquer thinner to get the varnish off, and change the cam and lifters and push rods. old gas can make a big mess in the valve train.