Had this same problem last week in one I worked on , took me 5 minutes of troubleshooting and fix. The point gap was way too wide. Don't ask me why it picked number 4 cylinder but after narrowing down swapping plugs/ cap, etc I went to the points and that cured the problem . If it looks electrical it is.
If you don't have a good spark on 4 it is electrical
Larry Shepard