What type of batteries does the cat flap require, and how do I know when they need replacing?

The cat flap requires 4 high quality alkaline AA batteries (LR6). When the battery is low, the red LED will flash every 10 seconds, indicating that batteries need to be replaced soon. When new batteries are installed, the green LED will flash 3 quick times and the cat flap will enter Automatic Mode. (Page 7)