Well, it is cool that you've experienced the running-on-one-cylinder right next to firing-on-all-cylinders so you have a good feel for the difference.
I was hoping your problem was fixed but didn't say anything before because I know that switching from BMR2A to BMR4A wouldn't/didn't fix the problem. In fact, I would guess that the hotter plug was installed to deal with fouling due to rich idle at elevation.
I was hoping that the
process of changing the spark plugs fixed the problem. Something like not having the wire clipped onto the plug or something. Right now I am thinking maybe the spark plug wire has a break (just assuming that you didn't jiggle something else). So fire it up and start wiggling the spark plug wire for the cylinder that isn't firing to see if you can get it firing again.