Remember just as importantly as your lost speed due to injury is your loss of strength. This is going to affect every aspect of your jumps…stride length/frequency and steps in your approach, takeoff at the board, and a lesser time in the air due to the weaker takeoff.
Have you sufficiently rehabilitated your hamstring. You mentioned it was a tear…leading me to believe that 4 weeks may possibly be pushing it too much (if you're expecting to hit those same marks as prior to your injury).
What I'm saying here is to not push this too hard too soon…if you truly feel it is time to start working out hard on this hamstring then there are things you can do to get back to that former strength. What types of specifics are you looking for???