When I was in the airport I saw this sign and I took a photo because it illustrates how Italians say the number three with their hands.
But to show the number four they switch it up and raise the index, middle, ring, and pinky fingers without the thumb.
Then for five they show an open hand.
Italians might raise their thumb instead of speaking to indicate a number with a one in it. For example, if you ask how much something cost and the answer is one hundred, the person can respond by raising their thumb.
To emphasize the number, they might raise their thumb and then move it around in a circle. For example, if someone had to pay an expensive bill of one thousand euros and you ask how much they paid, they might just move their thumb around.
Now go forth and practice.