Congrats!!! You listed Hawaii, but might be a long trek for 4 nights, but we are familiar from 20 years of visits. . . We have stayed at many HI resorts over the years, but those that fall in a luxury or romantic category for us are:
Oahu: The Halekulani (Waikiki)
The Royal Hawaiian (We were married there two years ago.)
The Kahala (Waialae)
Turtle Bay Resort (North Shore)
The Disney Aulani (a gorgeous resort but not right mood for engagement romance. imho. :-)
Maui: Four Seasons Maui (Wailea)
Hana Travaasa (Hana)
Kauai: St. Regis Princeville