Sorry I can't help you with boat to Nusa Lambongan however for your 3-4 days perhaps you could go to a few of the historical and significant places in Bali.

I would suggest a trip to Kintomani (crater lake and volcano), The Mother Temple at Besakih, Goa Gaja (Elephant Cave), Gunung Kawi (an ancient site), Tanah Lot Temple, The Elephant Park (north of Ubud), The Bird and Reptile Parks.

Hope you have a wonderful holiday.

Catherine