BMW E90 318i N46 - Cylinder 4 misfire only when warm, tried everything I can think of

Hi everyone, I have a 2006 BMW E90 318i with the N46 engine (around 230,000 km) and I’m struggling with a persistent cylinder 4 misfire. The problem started as a rough idle only when the engine was warm. Cold starts were always fine. After some time, it developed into an actual cylinder 4 misfire (P0304). Symptoms: Only happens when engine is warm Rough idle with a "putt putt putt" exhaust sound Misfire is worst at idle and low RPM Recently it started happening while coasting/decelerating and even with light throttle up to around 50 km/h Check engine light flashes when it happens Almost no noticeable loss of power Things I have already done:

Replaced cylinder 4 injector → misfire stayed on cylinder 4 Swapped/tested ignition components (coils/plugs are basically new) no change Replaced intake manifold gasket no change

pcv system is new

The misfire never moves. It is always cylinder 4. My next step is probably a compression test, but I wanted to ask people with N46 experience: Could this be: A burnt valve / valve seat problem? A Valvetronic issue? Something electrical specific to cylinder 4? Something else common on the N46? The strange thing to me is that it is always worse when warm and mostly at idle/low RPM. Any ideas or similar experiences would be appreciated. Thanks