I would like to understand if there is any additional license needed for MACSec on EX4300 switches

MACsec is supported on Juniper Network EX4300 series starting from Junos OS 13.2X50-D1. You can confirm this information on the following link: https://apps.juniper.net/feature-explorer/feature-info.html?fKey=8406&fn=Media+Access+Control+Security+%28MACsec%29

Also, you will need to have a specific MACsec license for the same to work as needed:
 

A feature license is required to configure MACsec on EX Series and QFX series switches, with the exception of the QFX10000-6C-DWDM and QFX10000-30C-M line cards. If the MACsec license is not installed, MACsec functionality cannot be activated.

To purchase a feature license for MACsec, contact your Juniper Networks sales representative (https://www.juniper.net/us/en/contact-us/sales-offices). The Juniper sales representative will provide you with a feature license file and a license key. You will be asked to supply the chassis serial number of your switch; you can obtain the serial number by running the 

show chassis hardware command.

Reference link for this and more information about MACsec: https://www.juniper.net/documentation/us/en/software/junos/security-services/topics/topic-map/understanding_media_access_control_security_qfx_ex.html