You've got a blown or badly leaking head gasket between 4 and 6. You're getting enough gas/flame transfer between cylinders that one cylinder or the other is lighting the mixture off too soon, before the spark plug fires. I check for a restricted exhaust system by simply loosening the pipe flange at the exhaust manifold and doing a test drive while watching intake vacuum on an uphill pull. An engine that won't pull the hill easily with the exhaust connected will fly up the hill with a leaky exhaust, and have a lot more manifold vacuum.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!