Advanatages of a 4CA - the points open and close
Disadvantages of a 4CA - cam profile causes point bounce, you can't set timing individually, condensors fail easily due to heat, etc.
Advantages of 6CA - you can set timing individually, has a good profile to the cam, condensors are away from the heat
Disadvantages - good ones are getting hard to find and pricy. Springs are no doubt shot and need replaced.
6CA advance unit is longer to allow for the extra height on the points. You really need to change as a set. Watch the CW vs CCW thing. It does bite the unwary.
For the cost of a good 6CA unit, you can go EI. You have the 6V coils. Convert the bike, unless this is a mostly non rider to 12V. With the 3 wire alternator, it is easy to do. Just need one of the modern charge control units (I prefer Podtronics
). Hide the charge control unit, use a 4A AGM battery
(it fits the early battery
tray), add EI and some 12V bulbs (which are much easier to find than 6V). No one will know the difference, you will have a better electrical system.