This will give your audience the impression that you are a mathematical genius with incredible "lightning calculator" abilities, even though the skill you require is no more than adding or subtracting 2!
In effect you show four long square rods or blocks, each with 4 different digits printed on them, on all four sides. The four rods can be arranged to give several thousands of different combinations (6144 possible combinations !).
A spectator is asked to arrange the four blocks in any order they please and total them - you instantly write down the answer, even before they can total two digits of the 16 digit "sum". You can reveal the total even before they can enter one row of 4 digits on a calculator !
The effect can be repeated as often as you like, with different results every time. Very easy to do when you know how - a school boy could do it after reading the instructions. And yet to the uninitiated, it is a very baffling mystery.