How is Sonic 4 any more retro than Sonic Rush aside from the name

Well, aside from the graphics, the game returns to its roots by continuing the original plotline of the Genesis games.
It doesn't have a boost button, nor does it have any grinding and tricks. Also, the games boss acts have the capsule to free the animals at the end, just like the Genesis titles.
It also continues the adding of one power to Sonics arsenal of abilities in each new game. In Sonic 2 it was the Spin Dash, Sonic 3 had the Insta-shield and Shield abilities. Now this one has his homing attack.
It also takes out the extra characters, such as Blaze, Nega Eggman, etc, and sticks to the original few, Sonic, Knuckles, Robotnik (Eggman), and Tails. Maybe Metal Sonic as well, not sure at this point.
Another change in detail is that Sonic has his classic running style of having a circular blur of feet, rather than the updated and more recent running style seen in Sonic Rush and Sonic Advance 2 and 3.