4:3a

Your lips are like a scarlet ribbon: Here the author compares the woman’s lips to a scarlet ribbon. The word scarlet refers to a bright red color. In ancient Palestine red lips were considered beautiful, so the woman probably put something on her lips to make them a bright red color.

In some cultures people do not consider red lips to be beautiful, or red lips may imply something bad about a woman. If that is true in your language, you may translate in a more general way. For example:

Your lips are a glorious color.
-or-
Your lips are beautiful.
-or-
Your lips shine.

scarlet thread: scarlet ribbon: The phrase scarlet ribbon probably emphasizes the color scarlet, which is bright red. The phrase does not imply that the woman’s lips were thin like some ribbon may be. In your translation you may need to compare her lips to something thick. For example:

Your lips are as ⌊beautiful as⌋ fine scarlet cord