“At about 20 minutes into the presentation, Joe describes FT4 as being asynchronous, meaning that you can start transmitting any time you wish, rather than being synchronized to an accurate clock. That asynchronous method did not work out as well as expected in on-the-air testing, and was abandoned.
The version of FT4 that will be coming out on Monday is synchronous, on a 6-second cycle, and requires a more accurate clock than FT8. One thing that you can do now, before the program release on Monday, is to make sure that your computer clock is as accurate as possible. The Meinberg NTP program is recommended by K1JT for keeping your clock accurate https://www.meinbergglobal.com/english/sw/ntp.htm
You can check the accuracy of your computer clock by going to www.time.is