WHAT TO DO....WHAT TO DO?

I have a mortgage with Chase....I just got a "over night" letter from them, it is the second one I've got in the last 6 months....The first, they offered me a re-financing "deal" a 30 year fixed at 4.88% sounded, not so bad, reducing my loan from 5.34% to 4.88....OK, so now I get another "over night" letter from them, "because I didn't act on the first one" They are now offering a 30 year fixed, at 4.00%, again it sounds, not to bad, but do I jump on this one , or wait another 6 months..... and get a better rate? (they say no, financing charges)