I don't think there's a general way: if the irregularity is a "slight one" (and here we already may have problems to agree on what is "slight") then you can take averages: the width in the front side, in the bottom side and one or two in the middle.
But if your quadrilateral is highly irregular then "average" can be a rather misleading notion.
Now, what would you like to determine an "average" width for? Is it to determine the quadrilateral's area? If so you can subdivide your quadrilateral in smaller simpler forms, like regular quadrilaterals and/or triangles...