Shred Alarm question

Hey guys cool place, question for any Shred vets. I just startex yesterday and did my first circuit day. Curious some excersises i could easily do level 4 reps some only level 3, is it ok to always try to work towards level 4 by increasing the reps in level 3 until im ready for it, or should I simply hit level 3 reps and rest the remainder of thr minute? I think I will try and do the working towards as opposed to just hitting the number. That being said can I do some days at level 4 and others at 3, throughout thr program or must I do level 3 only foe the 12 weeks?