I would have expected them to skip the 4xxx series entirely, having already used most of it on the CLRVs, ALRVs and even PCCs. 5000 would be a nice place to start - once I had found out about the numbering of the current fleet I was quite pleased to be able to catch, say, 4003 from time to time.
All series of numbers are officially spoken for. As I argue below, I would have started with 4600 not 4400.
1000 is used by the hybrid Orion VII buses.
2000 was last used by fully rebuilt GM New Looks (previously used for Witt streetcars). Two remain officially on roster, but not in
revenue service.
3000 (3000-3027) is currently used by the RT cars, though early GM New Look buses were numbered in this series in the 1950s and 1960s.
Of the 4000s, 4000-4005 is used by the SIG built CLRVs, 4010-4196 by UTDC/HS built CLRVs and 4200-4251 are the ALRVs. There's also 4500 and 4549, the two PCCs on the fleet for charters and special general revenue runs.
5000 is used by the subway fleet (T1s: 5000-5371, H5s: 5670-5807, H6s 5810-5935). Things get screwy with the Rockets though (ranging from 5381-6076).
6000 is currently vacant, last used by high-floor Flyers and non-lift equipped Orion Vs. However, the new order of Rocket Cars will spill over, leapfrogging the H-6s and maybe even the H-5s into the 6000s when the fleet is fully completed and delivered.
7000 are the oldest wheelchair accessible buses still used by diesel lift-equipped buses: diesel-built Orion Vs (7000-7134) and Nova RTS (7200-7251). From 7400 up, these are the older diesel Orion VIIs.
8000 is used by new low-floor diesel Orion VIIs, previously used for 1970s-1980s non-rebuilt GM New Looks
9000 has been the odd duck in the series, first for trolley buses, then CNG Orion Vs and VIs (the dieselized rebuilds maintain this number) and older Wheel-Trans buses. New W-T buses now start with a W prefix.
IMHO, the TTC should have started in the 6000s for the Rocket trainsets, since they started arriving after the last non-accessible Orion V was retired, instead of the mess of 5000s.
The new Bombardiers should have been numbered from 4600, to avoid colliding with the fleet numbers of the two PCCs. Even 4300-4499 would work for a fleet of 200 or less in a model series.