I live in MO and own 4 cars. For car registration fees, it wants the make/model, but my tax bill isn't broken out by car, it just has a total due. Any suggestions?

Yes.  One idea would be to take the value of each car, then divide it by the total value of all cars to get a percentage.  This percentage can be used on the total tax to arrive at the registration fees for each car.

Example: 

Car 1 Value = 10,000

Car 2 Value =   8,000

Car 3 Value =   5,000

Car 4 Value =   5,000

Total Value = 28,000

10,000/28,000 = 36% - This is the percentage for Car 1.  Follow this example for each car.

Next use the percentage for each separate make/mode to arrive at the amount for each car for your entry.

You can enter the total amount as if it were all for one car. Only the total amount actually goes on your tax return. The breakdown is just for your own records. And it doesn't matter what you enter for make & model. You could just enter "4 cars." It only goes on a worksheet in TurboTax, not on your tax return.