Short answer:
You cannot do that. If you want to save 4 byte character in mysql you need to use UTF8mb4
Longer answer:
In a hacky way, you can save characters like that with serialization. Say your model is Foo and column where you saving 4 byte character is bar, you will have something like:
class Foo < ActiveRecord::Base
serialize :bar
end
This IS hack though. It will work, but you will need to serialize all the other existing records (otherwise you're asking for trouble).
Been playing with ideas like that while trying to suppport emojis on legacy UTF8 database and unfortunately, other than opting for UTF8mb4, there's no other good way to go about it.
Another alternative if you're only interested in dealing with emojis is use something like emojimmy