Check your exhaust for a split cat system. Some vehicles have a pre-cat in each exhaust pipe before the Y junction to the main cat. It's possible the pre-cat could be blocked on the 4-6 bank side. A blocked main cat, or single system cat, would give trouble in all cylinders.
__________________
Good Luck...
mj
'01 Ford Ranger 3.0 Super Cab, K&N Air, MaxFlow Exhaust, Oversize Tires 
'93 Ford Taurus 3.0 (Ugh! What a MONEY PIT! Traded it in for the '01 Ranger) 
'87 Ford Ranger 2.3 Super Cab (Owned it for 16 years until some clown t-boned me in it!) 
'92 Ford Aerostar 3.0 (Gave it to Ex in Divorce Settlement) Hehe...
'74 Ford Maverick 250 ci (Great car! 110 mph on Autoban! Traded it in for the '87 Ranger)
'66 Ford Mustang 200 ci (Screwed up and SOLD IT!!)  | 
