Is the $4K for a week or a longer interval since you have almost a month off?
I think it's about $2500 just for flights from Canada's west coast to South Africa so that seriously limits your options. I haven't priced it but I expect Tanzania would be worse. You'll also take a bigger hit getting to the Philippines but once there things should be cheaper.
Also factor in the cost of boat dives - many islands run around 2/$100 and some don't have shore diving to offset that. And many of the places that do won't have the bigger stuff you want.
You could see the "big 3" on your list on the Big Island, Hawaii probably. Sharks are certainly present on many dives, the Manta Ray Night Dive is one of the top 100 dives in the world on some published lists and there's often spinner dolphins at some of the further south dive sites - my buddy did one dive there with about 500 of them. Plus the Humpbacks will be there at that time - not so much at dive sites but you'll probably hear them singing. Maui to a lesser extent has all 3 also. And you can puddle jump over affordably mid-week.
One advantage to either is that if you can hook up with some local people or other dive tourists you can shore dive there very affordably. A couple of shops - like Maui Dreams Dive Co. - have buddy sign-up lists. I beleive Jack's Diving Locker on Kona does also. Call ahead and get on the list b4 you go. And there's multiple guides on both available but that becomes almost as expensive as boat dives - which aren't cheap either.
Check vrbo.com for condo options on either island. Either in Kihei or south of Kona. There's some studio/1br. condos south of Kona for around $1000/wk. or less.
Of course it's high season in Dec. so you'll be paying a little more for everything - except boat diving as those prices don't change. Book a car before you arrive if wanted - you do on Kona anyway - as that can be substantially higher as a walk-in. And you have to do it about 2 weeks in advance with some companies or the rate is higher by several hundred dollars.
---------- Post added October 19th, 2015 at 07:50 AM ----------
The only shark dive on Roatan is a feed dive and the only dolphin dive is at AKR so maybe not what you're looking for exactly. Both are upwards of $100/dive also - even if staying at AKR. You will see a lot of rays, some grouper, turtles etc. Also the least expensive AKR room prices at $1300 Double occupancy and there's a Single Supplement. They have a BOGO also if your SO wants to go. AKR has enough other activities so they won't be bored if a non-diver and the West End or West Bay and other tourist attractions are a few minute cab ride - I believe AKR also organizes a shuttle for some of them. It is pretty isolated from anything else nearby though - the property is gated/secured and the Key is accessible only by boat. The bulk of the resort being on the shore side.
If Roatan has any appeal, look into something in the West End instead. It's probably substantially cheaper. You can get a decent 1br apt. with a small kitchen for around $50-60/nt. Older but generally clean and bug-free. Dives there are pretty much all boat dives at 10/$350 so that's a pretty good deal. There's a couple of small food stores, a lot of restaurants at all price ranges and you won't need a car - everything is walkable since the whole area is less than a mile long. Two miles away is West Bay, more restaurants/resorts and a nicer beach. The water taxi goes between them all day. Your fixed transport cost is $25 from the Airport.
Although we dove with/liked them also - Coconut Tree has a West End info page on their site.
Roatán, West End | Scuba Diving | Roatan, Bay Islands, Honduras | PADI There's also at least a dozen other operators in the same area.