69 Valiant cyl4 not firing?

Check the compression on all of the cylinders when you replace the spark plugs. You could have a burnt exhaust valve.

check the compression then squirt a little oil in the cylinder and check again.
If it was low and it improves = rings
If it was low and it doesn't improve = valves.

also check to see if your damper in the exhaust is not rusted shut, it is just under the carb. Sorry I forgot the real name of it.

Good luck