April is the beginning of Summer in Hue so it shouldn't be too hot. It's still cold here now, and March is sometimes cold too. 7-15 April is the Hue Festival so its a good time to visit (its only on every 2yrs).
I was there last May and it was very hot and very sunny. Not too humid. April is a good month to be there - both for the festival and the weather, which should be hot and dry (but not as hot as May).