But then is it not possible to have the contents of the scratch cards written down on a bit of paper & used as the reveal?
No. Sorry.
The whole effect is that, the 4 numbers that they freely select are the numbers on the pre-scratched card. It gets absolutely amazing reactions.
On the downside though, you are right, they refills are very expensive, and some have argued that the lottery scratchcard shouldn't be called "predicta".
Relatively speaking I haven't performed this a lot - I am on my third set of cards, and still use it, but due to the expense I save it for special occasions. It's too much expense (IMHO) to use at tables, but the reactions! Oh boy. It just seems so impossible, and is a nice keepsake.
As for it being called Predicta. That was a lot of fuss over nothing as far as I am concerned, all anyone ever remembers is that the numbers they THOUGHT of, were the numbers on a scratch card.