Some translations set each line of poetry farther to the right than the rest of the text to make it easier to read. The ULB does this with the poetry in 4:3-4, 7, which are words 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)).