I only have 2 risers and for cardio it works fine - I wear a heart rate monitor and I have no problem staying in my fat burning zone. For anything else like concentration curls I sit on my ball for more height - hope this helps!
I think you'll be fine. I only had 2 risers for 5 or 6l years before I finally purchased more and had no trouble with cardio or any weight training exercise done on a bench. I used my ball or my coffee table for any standing or sitting exercises that required more height. You'll still get a great workout.
Agreed, I have enough risers but like for Cardio leg Blast I only use 2. I'm a little scared to use more than that as I fear I may not jump high enough. I think you will be fine as well with 2 risers per side.
I only have 2 also, and agree that is challenging enough for me, i've been searching yard sales for extra risers because I think they are too expensive.