The National Hurricane Center uses the Saffir-Simpson Scale to categorize hurricane intensity by using wind strength within the storm. As of May 15, 2012 minor adjustments to scale will become effective.
The reasoning behind the change is due to mathematics. The Hurricane Center internally uses wind speed in knots when displaying current wind speed and forecasting a hurricane's intensity. For the general public the knots are converted to miles per hour and then rounded to the nearest 5mph.
A Category 4 hurricane has wind speeds of at least 115 knots and that will not change. In the past when a storm was a Category 4 and has a wind speed of 115 knots, that converts to 132.3mph. While it was considered a Category 4 storm, after the rounding to the nearest 5mph, it ends up becoming a category 3. In the old scale the rounded number would be 130mph. To keep it a Category 4 the Hurricane Center would have to incorrectly convert 115 knot wind to 135mph in hurricane statements.
This year the range in the wind speed numbers from Category 3 through 5 have been expanded slightly to keep things correct. Here's a look at the new scale from the National Hurricane Center
Category 1 74-95mph (Very dangerous winds will produce some damage.) Category 2 96-110mph (Extremely dangerous winds will cause extensive damage.) Category 3 111-129mph (Devastating damage will occur.) Category 4 130-156mph (Catastrophic damage will occur.) Category 5 157mph and higher (Widespread catastrophic damage will occur.)
This season when a Category 4 hurricane with winds of 115 knots is converted using their conversion criteria, the wind they correctly calculate (130mph) falls within the "new" Category 4 range.