4Runner should be fine as long as you understand that you're at the limit of what it can tow and you need to drive safely and defensively at all times. I wouldn't pull a 19' glass boat with a Highlander. 18' would be questionable. I would feel ok towing my 17' glass boat.
The 4Runner is RWD and has 2,000+ lbs more towing capacity than the Highlander, so those are the main differences I'm looking at. My wife has a V6 Highlander and loves it, but I wouldn't tow a 19' boat with it.
Life can only be understood backwards; but it must be lived forwards.
- Soren Kierkegaard