@ankit0810 When a visitor comes on to the website for the first time a cookie is dropped which identifies the visitor and gives the visitor a value, the visitor is a new visitor . Basically you are looking at the visit number of the visitors: if a visitor has at least one visit with a visit number higher than 1 (2 or higher) the visitor is segmented as "returning visitor" . It is not depending on the timeframe you are looking at.
You are talking about a custom varibale evar4 to identify , I assume this coming from your tracking implementation.
Generally , it is done through this plugin implementation :getNewRepeat
https://experienceleague.adobe.com/docs/analytics/implementation/vars/plugins/getnewrepeat.html?lang=en
Hope this helps