TE4 just reached an important milestone with the addition of 11 stadiums to reach a total of 20 stadiums !
Since last January, the other main additions are :
Reaching strike animations : hand-made stretching animations for the forehand & backhand when the ball is a bit too far and the player doesn't move too fast, giving more reach than the normal strikes, even stretched
new volley animations, including hand-made low and high volleys (that are still adjusted by the dynamic animation system so the racket is correctly positioned on the ball)
More robust animation synchronization for the CPU strikes and thus almost never fails
Other little things here & there, including gameplay tuning & enhancements
The game is getting more & more complete than TE2013 and offers now a lot more stuff than his predecessor.
Monte-Carlo stadium
As a reminder, here the current main improvements compared from TE2013 :
Better visuals with new 3D players, stadiums & NPCs
Reworked physics so everything is more realistic ; as the main example : now if you hit a ball when mispositioned and/or too late, it'll always have a chance to end in the net or go wild
New menu design
Men Tour starts in 1973 & Women Tour in 1983
Junior Tour
Future fictional years are generated in real-time, so each playthrough sees different top players in the future
Olympiads
3 new player styles to match more closely the global strategy of what I'd call aggressive defensive players (ie: players who base their game mostly on defense, but also can feel pretty aggressive)
Improved artificial intelligence
Dynamic animation system so the player always hits the ball with the center of his racket (as long as it's not too far from him, or really too much on the body)