If you can invest in your Roth at 8%-12% over that year and a half (yes, I know this is historical, and not guaranteed to stay), that will get you farther than paying down at a guaranteed mortgage rate of 4%. At the time you refinance, you could decide use some of that invested money to pay off more principal then (having still made more in the market, fingers crossed), or you can leave it invested and let it ride, and continue to (hopefully) outperform the mortgage rate with market returns.
For me personally, the math of investing extra money makes more sense than the emotional value of paying down my mortgage. However, there are two very opposite ends of that spectrum. You should go look for the two Pay Down My Mortgage and Don't Pay Down My Mortgage threads in the "Throw Down the Gauntlet" section. It could be worth some time to read both and see where your comfort level falls, and go from there.