> In both engines, my understanding is that the oil has to be used within the cylinders to keep them from ceasing, so surely a 4-stroke engine would also be burning oil too, with the oil just being added at a different stage?

Oil is not added because piston rings separate it from fuel. Rings keep the oil at one side of the piston (the crankcase side) and the burning fuel-air mixture at the piston head side. Ideally, the oil is not supposed to get to the head side at all: no mixing and no burning.

Of course, some is oil is left on the inner cylinder wall in the form of a film, some of it get washed away by fuel and burned by the combusting mix. But this amount is much, much smaller than the amount of oil that has to be mixed into fuel in cheap 2-stroke engines. The mixed-in oil passes through the crankcase just once yet it still has to provide similar level of lubrication as the oil that's (almost) permanently contained in the oil pan of a 4-stroke engine (or a proper2-stroke engine with a dedicated scavenging blower, as you've mentioned). That's why so much more oil needs is necessary when it's added to the fuel.