The sum of the digits in the unit's place of all the 4-digit numbers formed by using the numbers 3, 4, 5 and 6, without repetition, is :

We have the digits 3, 4, 5, and 6.
There are 4 choices for the units place. Since we are forming 4-digit numbers without repetition, there are 3 choices for the tens place, 2 choices for the hundreds place, and 1 choice for the thousands place. This means there are 4! = 4 × 3 × 2 × 1 = 24 total 4-digit numbers.
Each of the digits 3, 4, 5, and 6 will appear in the units place (1/4) * 24 = 6 times.
Therefore, the sum of the digits in the units place is 6(3 + 4 + 5 + 6) = 6(18) = 108.