If they are 4 months old you may have to wait a bit longer. But I do remember at a Frogday a few years back there were some for sale. The vendor misted them like crazy and then the lights went out for a talk, and they chirped like mad.
1) The call. Like the others mentioned, mature males will call. It sounds like a cricket.
2) Body shape. Females are generally larger and less streamlined than males. Their head is often less angular and they overall just look a little more round. In the picture below the male is on the left and the female is on the right.
3) Femoral pads. Males generally have very visible femoral pads. These are easy to see when one is climbing the glass. Take a flash light and shine it on the frog's hind legs and if it's a male you will generally see two white areas on the underside of the femur. Not as easy to tell as the other two ways.