I do not understand why 2 and 3 digit call signs have all but disappeared
Actually, the majority of United's own flights
are 1-3 digit flight numbers, with the low ones being mainly used on high-profile routes (e.g. UAL5 LHR-IAD). However 4-digit flight numbers are invariably used by commuter carriers operating UAL flights, with each partner having its own range of numbers.
For example on DCA-ORD, UAL's Airbuses fly as UAL207, 265, 287, etc, but on the same route UAL3461, 3463 and so on are United Express (Shuttle America) Embraer 170s.