Originally Posted by
HillRider
For road rear derailleurs 10-speed is backward compatible with 7,8 and 9-speed components.
Generally true, but there is
one exception.
Shimano's current 10-speed road group (Tiagra 4700) uses the new Shimano 11-speed road pull ratio. The Tiagra 4700 RD won't work with Shimano 6-9 speed shifters, or other Shimano 10-speed road shifters, for that matter - including Tiagra 4600.
I understand the same is true of the Tiagra 4700 double FD, but not the triple. Can neither confirm or deny this from personal experience, though.