If that were the case, the sensible response from Nichia would have been “let’s make more 4500K to meet the increased demand and make more money”, not “let’s discontinue it forever and lose this big customer right away”.
They certainly are available, such as this 4500K XHP50.3 tested by koef3. It’s somewhat sensible to not see them from distributors like Simon: a granularity of 500K is arguably unprofitable, as most folks would find a granularity of 1000K enough. Nobody seems to complain about lack of 3500K Cree’s, so if he did stock them, they might end up as surplus.
None of this really explains why Nichia wants to discontinue the 4500K though. My guess would be that 4500K almost never serves as a direct drop-in replacement for common sources, so maybe they didn’t see enough demand to justify continuing production. All the other CCTs are pretty much accounted for: 2700-3500K replaces incan/halogen, 5000/5700K replaces daylight, and 4000K for some odd reason is occasionally seen in LED bulbs, sometimes under the misleading description “cool white”.