The starting pitcher.
I dont want to be mean but, THE RELIEVING PITCHER!!!!
The pitcher who got the last out in the previous bottom of that inning. In other word, that pitcher can be credited with the win by making only one pitch. If I'm called in as a reliever and get a ground out to end the inning and then my team goes ahead in the top of the next inning, I would be the winning pitcher. Even if i only threw one pitch.
The win goes to the relief pitcher (assuming there was more than one relief pitcher) whom the official scorer feels was most instrumental in the victory. That relief pitcher must pitch at least 1 full inning, or pitch a crucial out if he pitches less than one inning.
Second to Third to First to Pitcher
In Major League Baseball there is no limit per game, but on the second visit in an inning, the pitcher must be replaced. Therefore, in a 9-inning game, a manager could visit the starting pitcher 9 times.
Pitch count has nothing to do with eligibility to be the winning pitcher. In a scheduled 9-inning game, the starting pitcher must pitch 5 complete innings to be eligible to be the winning pitcher. Relief pitchers must be the pitcher of record when the winning team takes the lead, and never relinquishes the lead, to be the winning pitcher.
With no addtional information available, the winning pitcher is the relief pitcher who came in in the 4th inning. Remember, a starting pitcher must pitch five complete innings before he can be credited with a win.
The winning pitcher is determined by the official scorer, if the starting pitcher's team is in the lead and never gives up the lead for the remainder of the game. If not, the winning pitcher is the pitcher of record when his team takes and keeps the lead. To be the "winning pitcher", the starting pitcher must pitch five complete innings in a game that is scheduled for six or more innings.
Unlike the starting pitcher, a relief pitcher has no minimum number of innings pitched. Given the definition of a win, however, it's impossible for a pitcher to achieve it without pitching at least 0.1 inning (one out).