This is one of the most effective practice techniques I have come across. As the songs you are learning have gotten longer, you have probably noticed that you are better at the first two measure than the last two. This is because when most people practice, they start at the beginning, play until they make a mistake, and then start at the beginning again. It is the musical equivalent of banging your head into a wall, in hopes of the wall being a little farther away the next time. Backwards practice turns this method on its head. You start by working on the end—in this case the last line. After playing through the end a few times, you start a little before the end (the third line in this song) and play all the way to the end. Instead of the frustration of going until you fail, you are gaining confidence as you pass through the less familiar material (the third line) into the material you have already practiced (the last line). When you feel like you are constantly succeeding as opposed to failing, it makes practice much more fun.
It does take some time to get into the habit, so just to be clear here is the way you should be practicing:
-Beginning of line 4 to the end (the end of line 4) 3xs
-Beginning of line 3 to the end (the end of line 4) 3xs
-Beginning of line 2 to the end (the end of line 4) 2xs
-Whole song 1 X
As always, it takes time to learn a song, so follow this plan and move on, even if it not perfect yet.