If the two boys are twins, and there is one birthday, I believe it should be Johnny and Stanley's 4th birthday. Two boys, one birthday. Then you could say "It is Johnny and Stanley's 4th Birthday".
If there are two birthdays (Johnny's is on the 6th, Stanley's is on the 14th but they're having a joint party because they're both turning 4 and we're sharing the cost of hiring a clown and renting a venue, for example), it should be Johnny's and Stanley's 4th birthdays. Two boys, two birthdays.
And I guess in that event, you shouldn't say "it is", but rather something like "come celebrate", since "it" is not one birthday. "Come Celebrate Johnny's and Stanley's 4th Birthdays!"
That's my two (or is it four, or perhaps eight, in this case?) cents.