To be honest after reading the ADF guide and how they need 5 gallons each I wouldn't recommend ADFs, they're also territorial at times, which is not good mix with a Betta.
You won't be able to go with my fish friends, community tanks need 10 gallons and up.
So you're left with shrimps or one snail, for the snail I recommend a Nerite snail, they're smaller which means less poop and they can't reproduce unless in brackish water, mystery/apple snails are grow too big and poop too much for a 4g.
Shrimps usually end up as snacks if you don't give them cover like some sort of carpeting plant, java moss or any moss plants works really well. You can start with Ghost Shrimps, they're cheap and if they die, oh well. If the Ghost Shrimps thrive try Red Cherry Shrimps, if they thrive they'll reproduce like crazy so they'll populate your tank in no time.
That's pretty much it. .