I own a 2015 Highlander V6. It's my wife's DD.
Get the V6. No question.
We test drove the 4 cylinder, and yes, it's adequate, smooth enough, and I'm sure will handle whatever you throw at it. But then we drove the V6 and it was a no brainer, especially since the MPG difference was insignificant (like 1mpg) and we were coming from a Camry SE with the the 2GR-FE. That engine almost seemed like overkill in the Camry. It was very powerful. In the Highlander, compared to the 4cyl, it just makes for a better driving experience. Non-arguably a smoother, quieter, quicker, and just as fuel effecient driving machine.
We recently took it on a 800 mile round trip drive and averaged 26.1 mpg, and that was loaded with 2 grown-ups, a 5 month old and ALLLLLL the things that going on a vacation with a baby includes
PS: just a side note: we love the Highlander. We stepped up to an SUV because of our growing family. It swallows cargo like a champ. It's the perfect step in height, good visibility all the way around, nice fit and finish, awesome middle seat with tons of room. 3rd row seat is tiny...def for kids only. But we usually just keep it down and load of the back. You made a great choice choosing the Highlnader OP, and if you can, get the V6. You won't regret it.