  We determine the structure of permutations avoiding the patterns 4213 and
2143. Each such permutation consists of the skew sum of a sequence of plane
trees, together with an increasing sequence of points above and an increasing
sequence of points to its left. We use this characterisation to establish the
generating function enumerating these permutations. We also investigate the
properties of a typical large permutation in the class and prove that if a
large permutation that avoids 4213 and 2143 is chosen uniformly at random, then
it is more likely than not to avoid 2413 as well.
