Marc, there are parties in Ibiza all year long. BUT the Ibiza you probably seek (I take it you've never been before) is the one that only kicks off at the end of May when all the big clubs open. From end of May to start of October, the nightlife is massive here.
As mentioned earlier, in April, of all the big clubs, only Pacha is open (and only on weekends). There are of course smaller venues and lots of bars and yes, as Craig correctly states, private parties too. Plus Ibiza is wonderful in April. But the nightlife is definitely only happening on the weekends and it's probably not what you expect.