What does "if there are previously drawn cards in your hand, rearrange the hand so that four cards are visible" mean?
In one-handed solitaire, your "hand" is all the face-up cards on the top of the deck. You are always supposed to have the top four of these cards visible, which can be achieved by fanning out the top four cards.

So this rule means that "if the number of visible cards is less than four, and the total number of face-up cards on the top of the deck is at least four, then fan out the top four cards so that you can see them all".