How is this true with no F4U4 in the lineup?
The F4U-4 was not a "long-range fighter intended for the Pacific". It was merely an upgraded Corsair. The P-47N was specifically designed for long-range escort of B-29's. If, in the case of my fictional scenario, the B-29 was to operate from much longer ranges over Europe, the P-47N was transferred to provide escort.
Don't read too much into this, please. For the most part, aside from a total reversal of some of the fortunes of war that occurred historically, this is a fairly plausible setup, even if totally fictional.