It sounds like head gasket between cylinders 3 & 4. All the compression is going out of the adjacent cylinder on the compression stroke. At low revs it will be ok as the engine will be under little load, as it is when doing 70 mph. Would be worthwile have the valves cut back in whilst the head is off. If it was rings, oil pressure would be high resulting from compression entering the oil system and or grey smoke from exhaust.