Once, I accidently underlined a 6wt with 4wt line and it cast like a cannon. It was a slow rod, however. Tight loops and all, I looked like a pro! I don't recommend it, nor do I recommend overlining, which slows a rod down a lot.
Your experience may vary, but it's worth a try since you have the line already.
As for line, I buy cheap line anyway. I've got so many reels, my individual lines don't get a whole lot of use. I bought a SA "bass" line at Academny Sports for $25 that's as good as any line I've owned.
