I think Tony Hinds just carries two car seats in his vehicle all the time.
If you would use him, you would need to contact and let him know you need 4 car seats.
I'm sure he would work with you if you gave him advance information.
For the other private companies, do let them know you need 4 car seats so they can be prepared.
Do you need 4 car seats or some booster seats for the car rides?