RE: running on 3 Cylinders
So it misses from 4k to redline? what about below 4k
if you are getting a spark in that cylinder then it is possibly a fuel issue. might be worth having a look at your carbs, if it's missing above 4k then have a look to see if the main jet is clogged in the corrisponding carb, if it is missing below 4k have a look at the pilot jet.
my 2c.