To get you back into it.
Chances are you bought the album because you liked the single. If you don't like 1-3, they reel you back in with #4 to cleanse the palate. Sequencing for albums used to look something like this:
Intro
Song(s) to get you hype/into it
Show(s) to show you skills
Single
Story rap
different style
another single
(maybe a dis song)
kick some knowledge
emotional song
maybe another single
posse cut
outro/last song