I won't facepunch you. I don't really swear either, so I guess I'm an oddity here in those respects.
If I put myself in your shoes, I guess I would think about a couple of things. First, how secure is my income stream? I would need to be really secure in my income stream before doing something like this. Second, I would look at how close I was to getting rid of PMI. If taking that $20K would get rid of PMI immediately, then I'd be pretty tempted. Third, I would consider income taxes. You're probably getting a federal and state tax deduction on the 5.125%, so your effective after tax rate is actually probably in the high 3-ish%. Paying 4% lump sum after tax may not even make financial sense in the first place.
Oh, and I'd also have to be very confident in my self control, to make sure I executed on my plan perfectly. I'm half-Vulcan that way and can do it, but almost everyone I know would get distracted by some other financial goal, change their minds, buy some more stuff, or do something else to screw it up.
I get offered CC deals at 3% pretty regularly, so you may want to wait until they offer you something like that, or even apply for a 0% balance transfer CC and hope to get a big enough line to do what you're talking about.
One other thing to note: Taking the whole credit line in a cash advance will trash your credit score for a while due to a high credit utilization percentage. I'd want to make darn sure that I wasn't going to borrow anything else for the next 15 months.
Finally, at some point you hopefully will realize that the principal balance matters a lot more than the interest rate. Shifting $20K from 5.125%+PMI%-tax deductible% to 4%/15 months is only probably going to net you maybe $1K, and there is hassle and risk involved. That $1K is 1/3 of 1% of your outstanding balance. In other words, you're playing at the margins really with this idea. Which I used to do too, so I won't say it's a bad idea. It's just not really going to change your financial future much one way or the other.
Good luck with whatever you decide to do.