Can't you split that up?

NO!
You have to do all 100 before changing exercises. Really.

"Angie" - For Time (Do 100 reps before changing exercises)

• 100 Pull-ups
• 100 Push-ups
• 100 Sit-ups
• 100 Squats

Results:

• Seth - 22:54 (6:59,7:11,4:54,3:48), as Rx'd
• Todd - 19:50 (7:10,4:20,3:00,5:20), scaled to 60 reps of each, 50# weight-assisted pull-ups

Previous results here, or more recently for James, here.