re: What to do with 1 year increased pay?

I think ive settled on putting a lump sum towards my mortgage principal to remove PMI. Now the question is should I recast the mortgage while I do this?
I believe if I recast, my mortgage principal + interest payment drops from $1530 to $1383 per month, so a savings of $1764 per year. I also save another $1896 per year in PMI by making the lump sum payment, regardless of whether or not I choose to recast. The recast fee is $250.

I'm not understanding the math on how to determine whether I should recast and reduce the monthly payment or just keep my same amortization schedule and pay off the mortgage earlier. I only plan to stay in the house for another 2-4 years. Can anyone help explain how to calculate this difference in interest?