Answer
You can form 24 tetrapeptides from 4 different amino acids.
Work Step by Step
If you have 4 amino acids you can arrange them in 4! (which means "4 factorial") ways in order to do a tetrapeptide, as a tetrapeptide has to be made of 4 amino acids.
And 4! = 4 \times 3 \times 2 \times 1 = 24
