Plain and simple: If you can walk 4 miles you can, and must, do this trail. The elevation gain is gradual but for a few steeper steps, and the stream crossings are a matter of stepping over 3 or 4 flat rocks to keep your feet dry. If the trail is too muddy, you'll know in the first 1/2 mile or so and be able to turn back.
Other than that, this trail is spectacular! From the banyan tree to the bamboo forest with its boardwalk; from the smaller waterfalls along the way to the 400ft beauty cascading down nearly on top of you, this hike will bring you closer to Hawaii than you can imagine.
We saw people of all ages and sizes doing this trail, so at least check this trail out and judge for yourself.