A back of the envelope calculation will get you further and be more understandable than most convoluted lengthy analysis.
You owe $130K. Given the small amount of principal paid in the earlier years, esp with a 30 year mortgage, the savings of 2.5% is $3250 over the first year.
(For those who think this too simple, the balance after 12mo at 30yr 6%, is $128,404, not even $1600 in principal paid, so the average balance over that 12mo is more like $129,200, and the math will show my 'envelope' to be off about $20.)
Geo - remember, the different payments you might see for 30yr/20yr/15yr mortgages do not save you anything (different). Cash flow changes, to be sure, and for some, that certainly comes in to play. You might also consider a no point/no closing cost mortgage, which might have a slightly higher rate, but at no cost at all, it's a no brainer.
Note - my comment above is strictly for the evaluation of the refinance. Assuming a 4% rate, 30yr will have a $620.64 pmt, 20yr, $787.77, and 15yr, $961.59. By taking a shorter term mortgage, you reduce the time and of course you save years worth of interest. That's a valid discussion, and I'd also concede that the 15yr mortgage faster amortization puts my envelope math off by closer to $70. i.e. if you saved 2.5% on the new loan, first year savings is closer to $3180, not the $3250. a difference of a few days to when you'd break even on the refi. If the 15yr payment doesn't bother you, you should see a rate a bit lower, as much as .5%, in fact. This will increase your first year savings, and reduce the breakeven time, just do the math.
If there are any closing costs, the $3250 savings in the first year gives you an idea how quickly you'd break even. Odds are it will be well under a year. For what it's worth, I am on my final refi, 3.5% 15 year rate, the cost this time will be about $2,000 on a $275K loan. All prior refis have been 0/0 deals.