Originally Posted by
UAzip
There are flight numbers with 1, 2, or 3-digit numbers (<1000) and there may be sCO/sUA issues in numbering, but also the whole range up to 9999 is not available. Express carriers get certain ranges each. Charters have a range. Codeshares have ranges by carrier. The actual number of flight numbers available for UA/CO metal isn't as large as it may seem. (This does not preclude on-time data shenanigans being possible, of course.)
There may be additional factors that limit the use of some flight numbers as well. For example, unless something has changed, there used to be no flight numbers ending in 13. So that can take a whole wack of numbers out of the system. Also, not sure how the China flights are numbered, but I doubt they'd risk using something with a 4 in it since the Chinese generally consider it a bad luck number. So if they are trying to sequence flight numbers into PEK/PVG, there's another that might not get used.