Well....
They both have to be adults(obviously). And they both have to be at least 4 years old. My tamas have 4 hearts too, but one of my tamas is an adult that's 1 year old. They other is 5 years old. Weird.
After they're both 4 years old, have ♥♥♥♥ in relationship(can't find another word for it), then just connect, and they'll do their thing.