If you look in a typical English Bible, Joel has three chapters. If you look in the NAB it has four. So what’s the deal? The NAB follows the Hebrew numbering system, most other English translations follow the KJV system. So here’s a little explanation of the difference by the commentators at the Net Bible:

• Beginning with 2:28, the verse numbers through 3:21 in the English Bible differ from the verse numbers in the Hebrew text (BHS), with 2:28 ET = 3:1 HT, 2:29 ET = 3:2 HT, 2:30 ET = 3:3 HT, 2:31 ET = 3:4 HT, 2:32 ET = 3:5 HT, 3:1 ET = 4:1 HT, etc., through 3:21 ET = 4:21 HT. Thus Joel in the Hebrew Bible has 4 chapters, the 5 verses of ch. 3 being included at the end of ch. 2 in the English Bible. (cite)

A sidenote or two: BHS stands for Biblia Hebraica Stuttgartensia, which is the standard critical edition of Hebrew Bible which scholars use. ET stands for “English Text.” HT stands for Hebrew Text.