as the OP stated, Hershey Park is only open 2 weekends in April - here is the info:
www.hersheyinthespring.com
I know in summer the 2 main hotels, Hotel Hershey & Hershey Lodge can be booked solid - I called once for a reservation 2 months out & they laughed, they can be booked a year in advance. I'm guessing since the parks aren't fully opened yet in April, it may be easier to get a room then.