You can check the cat like Fbird describes or with a vacuum gauge. If you drop the exhaust you can cause the O2 to see a lean condition and it would throw fuel control off. The vehicle should still run a hell of a lot better. When it was just cylinder 4 missing the cat seems unlikely, with all three on bank two misfiring it seems possible. The pending p0316 kind of discounts that possibility because the misfire seems immediate to start up and it usually takes a bit for a clogged cat to build pressure. I'm completely baffled so IMO it can hurt to check it. https://www.aa1car.com/library/converter.htm EXHAUST BACKPRESSURE CHECKS To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading at idle. Then increase engine speed to about 2,500 rpm and hold steady. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When the engine speed is increased there should be a momentary drop in vacuum before it returns to within a couple of inches of the idle reading. If the vacuum reading is 10 percent lower than normal and/or continues to drop as the engine runs, it probably indicates a buildup of backpressure in the exhaust. Remember, though, that intake vacuum can also be affected by retarded ignition timing and valve timing. What's more, some engines are much more sensitive to small changes in intake vacuum than others, so checking backpressure rather than intake vacuum may give you a better indication of what's going on.