You measured down 48 inches from the lowest point in the ceiling at the beginning. If everything went smoothly, your last row should fit nicely at the top of your wall.
Test a panel across the entire wall and make sure you don't need to make any additional cuts. If not, proceed as you have done for the other rows.
If you haven't already, go ahead and fill the bottom half of the wall up until you cannot place an entire panel due to the floor or baseboards and continue to the next step.