This is an effect by Michael Huot (from the 1st 7 By by David Acer) where 4 aces are shown and the spectator chooses any ace. You then show them that you knew they would pick that ace by showing the backs of the other 3 cards. Written on the back is 'I knew it' on all three. It gets a little chuckle but when you turn over the ace they chose it says 'I really knew it'.
David mentions other approaches to this such as 3 blue backed and the selection is red backed or you can have other messages.
Do any of you perform this? If so, do you have your own approach? I've been playing around with using 4 face down cards (jokers) having them select one and it turning out to be a previously selected card.
Thanks
Dan

Is the card the selected always have I really knew it on its back?
I guess trick could be done with 4 cards or 5.
IMHO, I like the comedy mentalism effects.
For example, 8 cards, all are queens. 2 of each suit. All shown face up.
They pick ANY one, You show your prediction : You will pick a RED queen! They pick a black queen, you pick up cards, and count all seven. They see all blue backs. They turn over the card they picked, it has a red back.
If they pick a red queen, you pick up other seven and show to have all red back.
Based on Killer Bee from Penguin magic though your post has me thinking...

Yes, the card they select will always say I 'really' knew it. Same for the colour scenario. The card they select will be always be red, the rest are always blue always.

Your idea sounds like a version of 8 card brainwave, correct? But I guess so is I knew it, just with four cards instead if 8. However, it cannot be repeated whereas 8 card brainwave could, theoretically, be repeated. Lennart Green has a really good version with a very nice handling for his display.