Reef or FOWLR? That will be the biggest determining factor.
For a 4 foot tank you will need to exclude the following:
Most tangs except the Smallest Bristletooth Tangs
Large Angels (might be able to get a small
Genicanthus in there ie Bellus)
Groupers, except for the Marine Betta
Large Butterflyfish
Triggers
Sharks and Rays
Beyond that you should be good to go outside of Reef Safe or FOWLR
I always have a good grazer, one small tang or a lawnmower blenny would work.
My 120g has clowns, chromis, anthia, tailspot blenny, royal gramma, mandarin, one tang.
Chromis and anthia may not be for everyone, but this is working for me, and I don't mind multiple daily feedings anthia need.
I personally would pick one of the dragonete's, 1 royal gramma.
I would not put a purple tang in my 120g, I prefer peace in my tank, and I'm not sure why it's a choice of that or a pair of bellus(which I think a 120g may be too small for)
I would pick a smaller and typically more peaceful tang
If you have not looked here yet, it might help you to look at LA list for fish compatibility, needs, and minimum space requirements.
They are usually fairly accurate, though their minimum do tend to be slightly lower(Helps to sell fish as more people have smaller tanks)
Also keep in mind while a 120g and 125g are almost same size, it's the length of tank that matters more for swim room especially w/ tangs.
Purples are typically very ticky and territorial, small spaces make this worse, so they may fit, but other issues may come about, like having problems adding new fish, and usually fussy sharing algae among other tangs.
I would be fine with me keeping Purple tang in 120. One smaller tang is great in a 120. Certainty avoid the larger tangs. They do tend to get larger than yellow. If you want angels, I would get Watanabei angel instead of Bellus. They are smaller for a 120.
Rather than 2 species of Dragonet, I would get a pair of one species. I love my Mandarin Dragonet, red, green or blue.
Fairy and Flasher wrasses are great, just stay away from some of the more aggressive Fairies.
Leopard wrasse harem is great also, if you have sand bed.