My hubby and I have five vehicles. His truck is a 1987 Ford Bronco 4WD. He had a few choice words about anyone who would let a vehicle sit that long without starting it. At the very least the fuel got stale and is probably formulated for the wrong season.
Here is the safety issue: if you had to, in ten minutes could you have all the kids and pets loaded up and be pulling out of the driveway? Well, with only one vehicle, if he is not home you can't.
Say things like, "Do you want us to burn up in a wildfire/drown in a flood/get twirled away by a tornado/get covered with lava/fill in your own natural disaster?
On the financial side, with three kids you could be putting that truck payment into their college fund.
We get the wildfires and the occassional flash flood and it takes me at least half an hour to get all the crates and dogs in a truck. Fifteen minutes of that is strapping Cody Newfoundland to the dolly and winching him up the ramp into the back of the truck.
Best of luck with your 4th kid.