Hi Marciadive, I was in Kenya/Tanzania/Zanzibar this past Feb myself. It was my first time on safari and it was fantastic. I dove in Zanzibar as well the neighbouring islands of Pemba and Mafia, about 5-6 days each.
In Zanzibar I stayed at Sunset Beach Bungalows in Kendwa, in the north west part of the island. The attached dive op was called Scuba Do. I also dove with a dive op called East Africa Divers in the northwest part of the island (15- 20 minutes by road) - better access to Mnemba atoll. Visibility was better and fish life was also better at Mnemba. But it is a good haul away from land - more than an hour if I remember correctly. The diving at Scuba Do was closer to land, 10-15 minute trips. Overall, both dive ops were good, but East Africa was probably the better.
Sunset Beach was okay, fine for me, I like just basic, clean a/c rooms, and that's what I got. Food and wifi was actually better at the hotel right next door. I'm not sure if East Africa Divers has a hotel (not attached anyway), but I'm sure there are some close by. I'm also not sure about the beaches on the north west side (for your husband), but I'm sure you can check that out yourself. Does he snorkel? There's snorkeling to be had. As well, land based activities in Stonetown, and spice plantations, etc.
Pemba was a disappointment but Mafia Island was as good if not better than Zanzibar. I stayed at the Mafia Island Lodge. Excellent food. Room was okay. The dive op was very good - highly recommended, very good bunch of people, they take good care of you . Problem with Mafia is the cost - $20 per day in park fees, whether you dive or not. And they do have non diving activities on the island. Plus they have a snorkel with whale sharks trip.
Other info: No ATMs in the north part of Zanzibar, so fill up in Stonetown when you land. The small planes used to fly to/from the island are really small - only one pilot, so one passenger gets to fly shotgun - excellent! (They don't mind if you video away) There is a bit of history to Zanzibar, so it may be worthwhile to keep a day for Stonetown, if that interests you.
Of course, 4 days are not nearly enough, but I'm sure you know that! Extend!