Answer by Skylinerw for How to detect when a button is pressed 3 times using commands

score_<objective>=# is checking for a maximum score, which means score_Tank=4 is looking for a score of 4 or lower. You'll want to use score_<objective>_min=# to check for a minimum score, so that it'll check for a score of 4 or higher:

/testfor @p[score_Tank_min=4]