Hi Senyor Pirotechnic,
I wish I could forecast the weather for April, it would make planning the camino so much easier... :?
I walked from Somport last year, end of March, and I had a really nice sunny day all the way to Jaca. The path was more or less dry, no mud, but I walked the first few km along the road as the camino was still covered with knee-deep snow. The next day from Jaca it was raining heavily, so I guess I was lucky.
I had the albergue in Jaca all for my own, then in Arres I met two other pilgrims, once again all alone in Ruesta, one other pilgrim in Sanguesa, and eight (!) other pilgrims in Monreal (and then of course hundreds in Puenta la Reina...).
So, in conclusion, the weather forecast for April will have sun and rain, maybe some snow. And there will be other pilgrims, though I doubt you will have problems finding accommodation.
Rolf