If the catalyst is blocked strongly with soot, the engine will even stall, but the cause of the exhaust clogging is the engine malfunction (high particulate matter emission). The question is, what are the values of the exhaust gas pressure difference at idle.
If, despite unblocking the exhaust, it still works on 3 cylinders, I would look for the reasons in the injectors or their installation (provided that the compression pressures are ok). Under the fuse cover on the engine, check that the injector harnesses are properly connected, that the plugs have not loosened and that the wires of this harness have not rubbed, especially when the harness enters the cover. Sometimes the nuts of the electric wires on the injectors under the valve cover can become loose and the problem is ready.
If the beam is in working order and no other damage is visible, it is possible that the injector is physically blocked and is not working, and if it is working and delivering too high a dose (leakage on the needle), then most often you can hear a strong knock in the engine, especially when it is cold. In such cases, the injectors to be removed.
