the fourth generation of 737 is called the 737 max. it is still being worked on, but when it is finished, it will be made up of the 737-max-7, 737-max-8 and 737-max-9. the max-7 will replace the 737-700, the max-8 will replace the 737-800 and the max-9 will replace the 737-900 and 739-900er.