Serpent Sting rank 1 tick will usually do 4 damage, 5 times, to add up to 20, and usually scales only off of spell power.
However, if you cast Chimera Shot to refresh Serpent Sting, it will scale as it does in WotLK, ~4% per tick. With 28 attack power on a level 6 character, my serpent sting’s first tick does 4 damage, then I refresh it with Chimera Shot and it starts doing 5 damage. If I then apply Hunters Mark, and Chimera Shot to refresh it a second time, it starts ticking for 6 damage.
This should probably either be reflected on tooltip if it is a ‘feature’, but I think it is likely simply an accidental bug caused by the way Chimera Shot was imported from Wrath?
EDIT: It appears this is not happening the same way for all hunters. Higher level hunters appear to not only not scale with attack power, but also refreshed serpent sting will do LESS damage. My level 6’s rank 1 serpent sting ticks for 6, while my 25 friend’s rank 1 serpent sting ticks for 3