It depends. If the issue is more your knee, going back to 1 and 2 might be good because the exercises are lower impact. If the issue is you are having trouble with workout 3 and 4 but 1 and 2 are fine, you might just need to do 3 and 4 for an extra week until you have mastered them: you need to force your body to change by doing the harder exercises once you can do the previous level. That being said, if it hurts: stop. Change the exercise so you can do it, but keep moving. Maybe instead of plyometrics, try holding a weight. Don't give up!

I agree with the previous posters. Perhaps you may decide to take a week off and start over. Let yourself heal then start weeks 1 and 2 again. In addition, regardless of the workout, just modify to what you CAN do. For example, once you're in workout 3 again, instead of doing the plank-ups, just hold a plank. If you can't do the full plank, hold a plank on your knees. If you can hold the full plank, try one plank up/down and then hold the full plank for 5 seconds, then do another plank up /down. With the step up/downs, do not use weights and allow yourself to brace yourself (helping yourself up); or just do them much more slowly. The key is to go at your own pace.

I have a lot to lose, too. On some of the exercises, I couldn't do them perfectly, use the same weights, or at the same speed as in the video. I always modify to what I can do, but always challenge myself. When I did weeks 3/4, I never was able to do the step up/step down with weights. When I finished week 4, I was able to do them at the speed in the videos, but just with no weights.

You can do this! It's your program....do what YOU can do and see yourself improve.