If Tavarez throws a 4-seamer it is a rarity and probably only when he is ahead in the count and the catcher calls for a high fastball. A sinker is just a two-seamer. It is more of a case of how the pitcher throws the ball if his two-seam will have a lot of sink on it or if it will flatten out. Pedro Martinez, for example, throws from a 3/4 delivery and his two-seam gets more left to right movement on it than sink where as Roy Halladay is more over the top and his two-seamer gets more downward movement. Whenever you see Tavarez throw a pitch that appears to sink more than his regular 2-seam it is either a change-up (which is around 84 and 86 mph) or he is just taking a little off of his fastball. A two-seamer usually sinks more when thrown at lower speeds, which is why a lot of managers/pitching coaches will tell you it is a good thing for a sinker-ball pitcher to be a little fatigued. The grip for a two-seamer and sinker are exactly the same though.
For further reference:
http://seattlepi.nwsource.com/baseba...tchingpage.pdf