Okay, so the formula is:
Payment = (I x Principle) / (1 - (1 + I) ^-n)
I = Interest rate at the time of the payment (in this case 19% / 12).
n = The number of periods.
Put it all together and you get:
146.62 = (0.0158 x 4000) / (1 - (1 + I) ^-(36))
So $146.62 a month.
@ soccerref: Just because something is commonplace doesn't make it acceptable. It is absolutely wrong to charge 19% interest on any amount of money. They would make the same profit by issuing three 6.3 percent interest loans of the same amount, and in the end it would be better for everyone.
you use an amotization schedule (readily available on line) or a financial calculator. The answer is approx. $146.62 per month. Total contract $5278.32.
Don't pay attention to the other writer, a 19 to 20% rate is very common for unsecured loans.