Some translations set poetry farther to the right than the rest of the text to show that it is poetry. The ULB does this with the poetry in 4:3-4, 7, which is from the Old Testament.
The word "rest" seems to refer to at least two things in this chapter. It refers to a place or time when God will allow his people to rest from their work ([Hebrews 4:3](../../heb/04/03.md)), and it refers to God resting on the seventh day ([Hebrews 4:4](../../heb/04/04.md)).