I’ve never had a Kindle with ads (though I do have a Kindle Touch coming in November that is ad-supported), so I don’t know how annoying it is to see them. But if they get to be too much for you, you can pay $30 (which is also the difference in retail price between the ad-supported and non-add-supported K4) by going to the “Manage Your Kindle” page:
Then click “Manage Your Devices” in the left sidebar . You’ll then see a list of your Kindle devices. If you have one with special offers, there will be a “Special Offers” column. Click the Edit button in that column of the device you want to remove the special offers from, then click the big orange “Unsubscribe now with 1-Click” button. Amazon will charge $30 to the card you have on file and you’re good to go.