Most odonata exhibit sexual dimorphism, i.e. the males and females don’t look alike, at least when mature. Unfortunately, this is not so with the Four-spotted Chaser (Libellula quadrimaculata). So, telling the sexes apart is a little trickier than normal.
Ignore the colour variation in the following; the female is very fresh and not yet as dark as she would become. Eventually, the female’s abdomen would look a little stouter but there, they are much the same. You have to look at the anal appendages which requires being at the right angle, too.