[quote=doofrat]
If you go from 4% to 8%, that’s 100%, so your payment effectively doubles.
At least that’s what I figured out, if I’m wrong, feel free to flame me.[/quote]

Not exactly…
There is a non-linear relationship between interest rate and payment. Although the interest portion would double, the monthly payment does not.
30-year loan:
400K @ 4% -> 1909.56 P&I
400K @ 5% -> 2147.29
400K @ 8% -> 2935.06 P&I

In this case, a 25% increase in rate yields a 12% increase in payment and a 100% increase in interest rate corresponds to a 53.7% increase in monthly payment.