April is out of season, although Easter will be packed everywhere (15.04) - it is the biggest religious and family event in Greece.
The weather will most probably be unstable, and you will find many places not open for the season (usually starts early May - middle of May). Except for the Easter ferries will still be on winter schedules.
If you go for staying on the islands you should plan your itinerary with choosing islands within the same group. In April you should also head as far south as possible