555 is supposed to be based on a cig British Tobacco called 555 State Express. Chinese developers first began this formula for juice back in the day. I have tried several and all are different.
RY4 (short for Ruyan #4) is a very popular eliquid that incorporates three basic component flavors: a tobacco base (usually mild and understated, but occasionally stronger and more up front), caramel, and vanilla.
see:
http://www.e-cigarette-forum.com/fo...views/193327-really-big-ry4-roundup-long.html
There are apparently as many versions of RY4 as thoughts in your head at bedtime.
Dxman