random number generator

i want to build a random number generator, that picks the numbers from which to generate from a list that i supplie, and i found this formula:

 =INDEX($B2:$B21,RANDBETWEEN(1,COUNTA($B2:$B21)),0)

the thing is that i need to generate numbers in 4 diferent cells, and i need them to be diferent

the attachment may be a little more clear.