I love 4D split. It fits so well into my schedule! It goes so fast. I feel like I'm getting a good workout. Each time I've done chest and back, I've gotten DOMS.
THe heavier weight makes such a difference. There aren't as many reps needed, but you definately can challenge yourself with the right weight. For me, I like the straight forward approach, cardio first and then weights. Each time I've done the workouts, its gotten smoother (esp. weight selection), and a better workout. For the weight workout portion, I feel like I'm getting a pretty good workout.
Hope that helps. Ditto - The premixes rock!