For a warm-up,  please refer to the 4 Minute Workout PDF page 6.

Bodyweight Warm-up Circuit
Go through the circuit twice and do each exercise at a 2-0-1-0 tempo.
1. Prisoner Squat – 10 reps
2. Mountain Climber – 6 reps per side
3. Prisoner Reverse Lunge – 6 reps per side
4. Pushup – 5 reps

You can also see a video with a warm-up here: 
http://www.youtube.com/watch?v=fXIKzeVEDFU

You can also download a warm up video from here:
http://s3.amazonaws.com/etr-dl/Trim-Down-25/c_Trim_Warmup.mp4