I am not a financial expert, but we have been through this process a few times. A good rule of thumb is that if you can lower your interest rate by a point or more, then it is a good thing. For example, given your current interest rate, your principal & interest payment should be around $590 per $100k borrowed (this excludes any insurance and taxes as that will not change with a refi). If you were to refinance to 4.88$, the P&I payments would drop to around $530 per every 100k borrowed. So, basically you would save $60 per every $100k of loan amount you have on your mortgage. This can really add up to big savings.
But, there is a catch you must consider. Refinancing is not free. It should cost around $2000 in closing costs (which I believe a part is tax deductible). So, you need to calculate how long you think you will be in the house to make refinancing worth it. For example, if it costs $2000 in closing costs and you are saving $200 a month with refinancing, then you need to be in your house more than 10 months before it becomes a "good deal". But after that you would be saving a lot for perhaps many years.
Lastly, be very wary of mortgage brokers trying to hide a good interest rate with making you pay down points. If you are not familiar with this, then just make sure you always ask about this. Paying down points is paying down your interest rate in cash. Any point percentage is basically mulitiplied by the value of your home and paid in cash upon closing of the refi. Sometimes this is a good thing as a little cash up front cash really lower the interest rate. But, lenders like the hide this and call it an "origination" fee to make you think you're getting a good interest rate. For example, your current mortgage could be disguised as 4.85& with 1 point down. So, you could end up paying to refi for the same exact mortgage you have! I would run for the hills of any lender wanting me to pay for than 1/8th of a point for anything above 4.75%. But, if your interest rate is 4.88% with no points down, you could consider paying down your interest rate more if it makes financial sense. All you have to do is ask and then crunch the numbers!
Hope I didn't confuse you! I still think that if you can get 4.88% with no points down, or like 4.75% with 1/8 of a point down it would be worth it. I think rates will be low for a while, but this is a golden time to lock in a historically low interest rate. My folks remember paying 12% interest of one of their homes when rates were really, high. Basically, anything under 5% is just a litte more than a standard inflation rate. In time, it will feel like the bank is paying you to have a mortgage :) So,good luck!
H.