Matt,
look at the levels at
www.eauxvives.org/fr/niveaux/en-direct/index
Unpredictable where the water will be in april, but expect no good water in the France Alpes, chance is high it is too early for that.
Better chances wiill be:
the Alpes-maritimes (in the list under PROVENCE-ALPES-CôTE D'AZUR). Search the web for those rivers and you 'll find enough in the area of class 3-4, also put in the Verdon and Var (allthough for them it also could be too early as they come from the more northern part of the alpes martimes, depends on the temperature)
The french Pyrenee (MIDI-PYRéNéES) could be good in that period and also contains a lot of class III-IV.
The massif Central (around Tarn and Ardech) can be OK, but have less snowy mountains, but there is snow over there so check out the levels.
cu, Menno