I do not know the exact odds, but here is my theory
If you play with 3 others (ie 12 cards already out) and no one is showing a low card, then there are 40 cards left in the deck.
There are 20 cards (ie 4 5 6 7 or

that you need 2 of from your 4 remaining cards to come.
This would suggest to me that 50% sounds reasonable, but cannot be more - likely to be a little less as some of the cards will already be out.
Mind you, from what ive seen playing this game on the net - not many others have a clue how to play the game!!