In the grand scheme, the Tiagra RD-4700 isn't a lot of extra cost compared to the STI's alone. And a front DR might be needed too. But I'd wait to see how well the ST-4700 did or didn't shift the FD-4600. Though maybe you others know for certain.
Even with the cost of two DR's added in, It'd be worth it to me to have the newer and so far most current Shimano 10 speed road stuff on my bike. I never got use to seeing all the cables flying everywhere. Aesthetics are much nicer with them taped to the bars. IMO