Chinese time is easy to convert from EST. Yes, I am Chinese. In addition, it's easy to search up from the internet. I also happen to know all of the US timezones, which I have memorized by now for my ease of using (as well as several other random timezones). So it is rather easy for me to figure out China time, even when I'm outside of China.
One last reason:
Btw, didn't you just state that you were staying up until 7 AM? :V And if you're saying it's 4, it means that you're currently west of China.