Well it does add 1 round of rage a day and +1 on fort saves.
But either you're confused about how hp work or I'm confused about what you write.
18 con doesn't give you an additional hp every 4 levels, it gives you 1 extra hitpoint per level over 17 con (equivalent to tougness).
19 con doesn't gain you a hp so you'd need to get to 20 con to get any more bonuses for pumping con which would take until 12th level. Which would get you 24 more hp than a 17 con would.
Whether that's worth it or you're better off pumping str is a separate question (I'd think that 1 point spent for 18 con could well be worth it but generally I'd be putting str up for a barbarian)
