How to sort four numbers in 5 comparisons? Efficiency in terms of space, time and number of assignments is not considered.

Algorithm: sort(a, b, c, d)
if a > b then swap(a, b)
if c > d then swap(c, d)
if a > c then swap(a, c)
if b > d then swap(b, d)
if b > c then swap(b, c)

subroutine: swap(x, y)
temp <- x
x <- y
y <- temp

The resulting sequence a, b, c, d will be sorted in ascending order.