Throwing the one in on the top as an extra. Not sure if it works but the 3 work no problem, 4th one might .

This Panasonic KX-TGEA20/KX-TGE230 expansion handset phone set comes with three black handsets and a base that is in working condition. The wireless system is compatible with landline phones and has features such as call waiting, noise reduction, redial, speakerphone, and caller ID. In addition, the set does not come with a manufacturer warranty.

The brand of the phone set is Panasonic and it falls under the categories of Consumer Electronics, Home Telephones & Accessories, and Cordless Telephones & Handsets. The number of handsets included in this package is three and each handset has three phone lines. The model of the phone set is KX-TGEA20/KX-TGE230 and it is perfect for those who are looking for a reliable and efficient cordless phone system.