For a cylinder to function as it should, it needs three things, compression, fuel & spark, all of which can be checked without removal of the cylinder head.
You say that you "cleaned the carby" but what does that mean? It got a squirt of degreaser & a hose down, or it was removed, dismantled, chemically cleaned & rebuilt with a new gasket kit?
Did you check the plug leads with an ohm meter? Are they within specs?
Have you closely examined the inside of the distributor cap? Have you switched the plugs from the good cylinders into the dud ones & checked to see if the problem traveled with the plugs.
Have you tested the plugs in a test machine, or just looked at them.

Did you set the points with a dwell angle meter, or a feeler guage, or just by eye.
A vacuum guage is also a wonderful diagnostic tool & should be used if available.

If a dry compression test shows that the pressure is at least even between the cylinders, even if it is a little low, then that isn't the cause.

If two cylinders are working & the other two are not, & it has a single carb, then it isn't likely to be fuel system unless the gasket at one end of the manifold is so badly damaged that it's sucking in a lot of air into two adjacent ports, so that just leaves the spark.

In the end, it's just a machine & we have the technology to diagnose & repair & the Gregories repair manuals are a 'must have' in every home mechanics toolbox.