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You are asking her to be a father? How about a mother? That would be the natural thing to be... unless I am missing something... :?[/quote]

Kalinka, stop being a slowpoke :), Elena is Mitya's...

Да
Я знаю Елену почти четыре года
Я знаю Елену вот уже почти четыре года
Скоро будет четыре года, как я знаю Елену

If you prefer to be precise on the matter you may say:
Я знаю Елену (вот уже)...

But don't forget to ask her in private.
Get her definite "yes" on the matter before announcement, and ask her is it ok to announce that.
Just to make some thinks easier for you :)

But anyway -...