re: Better to finance $31,800k at 4.19% or $35,100 at 0%?

Assuming equal terms, you are correct ~$203.

Will you be upside down on the loan if borrowing $35,100? If so you need to think of GAP insurance, which could cost $650-1000 defeating the purpose.

There is usually a fee to refinance ~$100. Maybe get pre-approved else ware for lower then the %4.19, get full disclosure of the new lein holder refinance fee and do some math opposed to making assumption of the availability of a lower rate