Couldn't you just practice the skill without caring about what level it is at? If you go to "Practice" on the top bar and choose Math, Reading and Writing, or Essay, you can then scroll down and see all the skills in that category. The questions you do will count towards your progress no matter what level you were at on the skill before you started.

Let me know if I'm missing something about your question.

Yeah sure, I know that.
Let me rephrase, I meant that once you get to the 4th level on every skill, it’s useless to keep on practicing the same questions and would be great to go back to previous levels to review the basis of every skill; and since now you’ve to get many questions wrong in order to do that, that option would be useful (at least for me)
Thanks for your answer!

I see what you mean now. No problem!