Cylinder number four has no pressure

I would start with a cylinder leakage test. A compression test tells you there is 0 psi in that cylinder. A cylinder leakage test will tell you why.

You put that piston at top dead center on the compression stroke, (there is a whistle that can help you find it faster), then the test involves pumping compressed air, through the tester, into the cylinder. The tester will show the percent leakage, and you can check in four places to identify the cause of the leakage.

Hiss at the tail pipe: leaking exhaust valve.
Hiss at the throttle body: leaking intake valve.
Hiss at the "oil" cap or dip stick tube: leaking piston rings or hole in the piston.
Steady stream of tiny air bubbles in the radiator: leaking cylinder head gasket or cracked head.

Thank you very much for your reply!! That sounds like too much special gear for me to do! I guess it will end up in the knackers yard.

It is somewhat involved, but you may find the tester at an auto parts store that rents or borrows tools. The most time-consuming part is finding top-dead-center. There is a whistle for that which is screwed onto the tester's hose. When the valves close, the whistle whistles as the piston is coming up on the compression stroke when turning the crank by hand. It stops whistling when you're passing top-dead-center. Then it's just a matter of looking at the timing mark on the vibration damper. You might be able to do the test if you're just close to top-dead-center, but if it's off too far, the compressed air will push the piston down and open a valve.

Once you're at top-dead-center, it just takes a few seconds to remove the whistle and connect the tester. As I recall, the tester regulates the compressed air to around 30 psi. One complete crankshaft revolution will put a second piston at top-dead-center of its compression stroke, so that one is easy to find, then a half a revolution does the same for a third cylinder.

In your case you're only concerned with cylinder number four. Other than finding TDC, the test doesn't take much longer than doing the compression test.