Were the 2 ignition coils and 2 spark plug wires replaced too? If not then replace them before going any further.
There is no separate ignition module. It is part of the ECM. It almost never fails and shouldn't be considered unless a last resort.
Chances are the "pre-heat" cat convertor, which is bolted directly to the exhaust manifold, has collapsed internally and is blocking the exhaust. Only way to find out is to unbolt it and check it. At the same time you MAY find a crack in the exhaust manifold by #4 cylinder. The only fix for this is replacement.
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