I know, I know. I looked at verse 2 yesterday. But it bridges verses 1 and 3-5 in such a way that I think it needs to be included in the discussion of both. Verses 3-5 talk about guarding what we say, and verses 4 and 5 specifically talk about promises.
Many are the fools who say things that they shouldn’t before God. They offer up illegitimate worship or make promises they don’t keep, and in both cases they are better off not saying anything. The second half of verse 5 says, “A fool’s voice is known by his many words.”