If birds are the main subject, there is simply no substitute for the 150-400. It is essentially a one-lens solution. However, it is expensive, heavy and not as flexible as the 50-200 plus a couple of TCs on your belt loop.
The 100-400 has slightly better IQ than the 50-200 +TC2.0 if you pixel-peep but the 50-200 combo shoots at 50 f/s vs 25 f/s for the 100-400. The 300F4 is better than the 100-400 unless you put a 1.4TC on. Then it is about equal but will shoot at 50 f/s.
I will say that the 50-200 with or without a TC is simply a joy in my hands. I find the smooth MF ring much better than either the clutch setup on the 300f4 or the MF ring on my 100-400.
The 50-200 with a 1.4TC is now my walk around lens, supplanting the 100-400. I have a 2.0TC on my belt loop when I really need it but I hate separating the camera from the lens in the field. Th semi-macro is simply awesome, so bugs or birds is my new mantra.
I use the 150-400 when I am not walking much. Those are my two lenses, and I don't expect to purchase another OM System lens.