Let's do a simple example to explain ----

400K loan, started with 5% on 6/30/2000, monthly pay :2147, for the last 10 years.

Now 10 years later, we will have Principal = 324576

If stay with 5% rate, and pay 2147 monthly, it will pay down on 6/30/2030.

If refinance with 4.75%, and still pay 2147 monthly,it will pay down on 9/30/2029, almost 1 year earlier.

If refinance with 4.5%, pay same 2147, paydown on 2/20/1029, abuot 1 and 1/2 years earlier.

Even with 4.875, you can still pay down half year earlier on 1/30/2030.

So, if it is .25% lower without any cost, and you got time, i think it would worth to do it.