Lending Question. How important are fixed rates?

[Garrett Hollenbach]

Hello everyone, 

I have been shopping around for lenders to help me with my refi. Most offer the usual 70%-80% LTV at around 5% for 20 years. But I found a lender that would give 80% LTV at 4% interest for 30 years. I was really pumped until I found out that the rate isn't fixed. In fact, it changes every year. How dangerous is this?