If removing the #4 plug makes not difference then its running on 3 cylinders..... which you should really be able to tell to start with! Does it sound sort of like a WRX when you rev it?
A dropped cylinder can be caused by a few things:
No fuel - But this is very unlikely for a 4kc given all other cylinders have fuel.
Lack of compression - Either a stuck valve or dodgy rings can cause this. Doing a compression test will quickly confirm or deny it.
No spark - This is the most likely cause. Could be a dead spark plug, faulty plug lead, or maybe even the dizzy cap. Try swapping a plug and lead from another cylinder and see if the dead cylinder is still #4 or the one that you swapped the suspected faulty plug/lead onto.
Its probably not a bad idea to just re-build your whole ignition! I know that might sound intesive and expensive but its not really. If you head down to supercheap auto you should be able to get:
New NGK plug
Bosch supersport plug lead set
New point and condenser
New rotor cap and buttom
.....for for under $50!!!! I don't mind buying that sort of stuff from SCA as its all branded.
Replace all of that, re-set your plug gaps, points and timing and if you're still only running on 3 then I start looking at the stuck valve scenario.