It's broken, but better

A merchant had a forty-pound measuring weight that broke into four pieces as the result of a fall. When the pieces were subsequently weighed, it was found that the weight of each piece was a whole number of pounds and that the four pieces could be used to weigh every integral weight between 1 and 40 pounds.

Let the weights of the pieces be $a,b,c,d$ such that $a\leq b\leq c \leq d$ , what is the value of $\overline{abcd}$ ?

If $a,b,c,d$ were $1,4,23,56$ , input your answer as $142356$ .