You are about to meet the most surprising - and beneficial - of all the basic finger-picking drills. It is exactly the same as exercise 3, except instead of each finger getting it's own string to pluck, they are both going to pluck the same string!
If you're like almost everyone else in the world, you will be astounded at how much more difficult this is than the previous exercise. Why is it more difficult? One reason is that your fingers now have to stay out of eachother's way - the thumb cannot touch the string while the index finger is playing it, and vice-versa.
As wierd and useless as this exercise may seem, it teaches a fundamental skill that you will really benefit from if you are trying to play a wide range of music. In a solo, a lick, a phrase, or any other piece of music, you may need to play a number of notes on the same string. While the fingers of your left hand will be setting the strings to the appropriate notes, your right hand will be doing this exercise - sometimes a couple of notes, sometimes a great many. Doing this drill will make it much, much easier.
Next, we're going to take a short break from the non-musical drilling exercises, and see a specific example where this exercise shows it's relevance. Are you ready to play a phrase from the hit song Wipe Out on your ukulele?