If they'd have called it TB6 there would be complaints about the fact it's not a byplane, and TB7 would have elected questions about what's TB6.
TB1-5 are rescue machines, having a security related ship (or whatever Kayo's ship will be) having the same registration would be a bad PR move.
As for Gordon in the second half (or second episode if you will)
John: Gordon, I'm picking up a distress call from a fishing trawler taking on water
Gordon: Tell them I'm on my way
However we don't get to see a TB4 launch, nor was it taken to the danger zone by TB2 (which took pod 3 with the rollercoaster seat attachment), and that's the last we see of Gordon until the mass evacuation after the threat of Grandma's cooking - as if 5 minutes was cut from release.