The specific answer to this question is...it depends what island you are visiting. But the general answer is...go now! March and April are considered dry and comfortable months. However, weather conditions fluctuate so greatly sometimes even within one island itself, that determining when the best time to visit becomes a matter of preference, cost, and weather.
Our friends at hawaii-guide.com give a nice summary of the nuances you'll need to keep in mind when planning your trip. You can find more information here.
Of course, whenever you're island hopping, make sure to shop with Hawaiian Tropic Apparel first!