Flightaware hasn’t been picked up on them yet but should by about 5:00 AM CST tomorrow. Example a BWI-SAN that will depart at roughly 3:00 PM CST will be carrying a 5418 flight number.
Just to clarify: these are regular flight, nor charters or ferry flights or maintenance flights.
There was a Southwest blog a while back about them having to really check out a lot of things so that the flight numbers 3000 and above were okay and wouldn’t screw up things.
do you have a link to that blog post? I did some searching but couldn’t find it.
Took me a while but I found it: HO HO HO, WHO WOULDN’T GO…MILWAUKEE SERVICE AND HOLIDAY TRAVEL IS NOW OPEN FOR SALE!
Relevant portion (2nd to last paragraph)
For example, up until now, the highest flight number we could use on a normal schedule was Flight Number 3999. By starting from scratch for these three days (in addition to the standard Monday and Saturday optimizations), we quickly ran out of flight numbers. So we upped the flight number limit to 5999–which sounds so innocuous, but you’d be amazed how much testing we had to do with dozens of other, internal computer systems to make sure they could handle flight numbers in that range!
You forgot to tell them to use the search feature and post this in the Southwest thread and Google is their friend. Just kidding about that last one, Google is nobody’s friend. 
I don’t see why they need flight numbers so high. It’s not like they actually have 5000 flights, especially with how many through flights they have.
In fact, all United operated flights are 1, 2, or 3 digit numbers, reserving all 4-digit numbers for United Express or Codeshare.
Delta, on the opposite side of the spectrum, is for some reason using 3-digit flight numbers for the Mesaba operated Saab 340 Delta Connection flights out of ATL, numbers that were normally used by mainline flights.
If you study the Southwest timetable you’ll see that most flights fall into one of 5 frequency groups: Weekdays only, Saturday only, Sunday only, daily, and daily except Saturday. The majority fall into the first two groups.
It is easier and more logical to have each flight that operates on a different frequency assigned a different flight number. For example, there’s a flight from SMF to LAS that departs at 07:30 Sunday through Friday. The Saturday flight leaves at 07:35. It doesn’t make any sense to me to have the daily except Saturday flight numbered the same as the Saturday flight. (source: southwest.com/pdf_schedules/ … mplete.pdf page 501)
United and Alaska are the only airlines in the USA that doesn’t use 4 digit flight numbers for mainline flights. The other legacy carriers (American, Continental, Delta (including Norhtwest), and US Airways) and the major carriers such as Airtran, Southwest, JetBlue, FedEx, and UPS all have 4 digit mainline flight numbers.
According to the latest Delta timetable, flight numbers 850 to 947 are operated by Pinnacle and numbers 950 to 999 are operated by Mesaba. These were Northwest Airlink operators so this may be a special case and once the merger between Northwest and Delta is completed the flight numbers may revert back to 4 digits.
You would need to have some time on your hands to get thru the whole thing…like maybe taking the slow boat to China…
As far as damiross’ statement about charter/mx/ferry flights, I think they are all 8xxx/9xxx numbers now. Although, if you have a certain issue of Flying, where Dick Karl joined in on a delivery flight to PHX (ship #454), I believe that flight number was 5001. Things change, I guess 
That’s a great story. Dick must have looked like a golden retriever going for a car ride.