For each regular tetrahedron, the probability of four faces facing down is 1/4, so the probability of one face facing down is 1/2.

p(x=0)=( 1/2)^4= 1/ 16，

p(x= 1)=c(4, 1)*( 1/2)^4= 1/4，

p(x=2)=c(4,2)*( 1/2)^4=3/8，

p(x=3)=c(4,3)*( 1/2)^4= 1/4，

p(x=4)=( 1/2)^4= 1/ 16 .

The list is as follows

x 0 1 2 3 4

p 1/ 16 1/4 3/8 1/4 1/ 16