Time to make friends with an amortization schedule. They are available online, just google it up.
You want to figure out how long you'll be paying on the note, and then you can figure out if that $3k is worth it or not.
Oh, what the heck.
So, with a $300k mortgage, monthly payment (just P&I, which is all we care about here) is:
$1472 @ 4.25%
$1494 @ 4.375%
$1516 @ 4.5%
The 4.375% loan will gain you an extra $1640. Divide by that $22/mo and you get a payback time of 74 months. The 4.5% loan will gain you $3855. Divide by $44/mo and payback is 87 months.
So if you are planning to pay off the entire loan faster than 6 years, take the money and run. If you are positive you'll stay in the house and it'll take longer than that, pay extra for the lower rate. At an extra $500/mo, you'll be paying off in 216 months, so the lower rate/higher closing cost makes sense. To get down to <74 months, you'd need to be paying somewhere around $4700/mo (an extra $3200 or so).
-W