Can we customize kibana 4?

Is it possible to customize kibana 4 for using it according to my need ??

That depends on the nature of your needs.

How can we do that ?? i mean if i want to customize kibana 4 where should i make changes?

Can i embed kibana 4 graphs or functionalities into another web app?

How can we do that ?? i mean if i want to customize kibana 4 where should i make changes?

You can of course make changes to the source code and rebuild. Depending on the kind of customizations you want to make there might be shortcuts.

Can i embed kibana 4 graphs or functionalities into another web app?

See https://www.elastic.co/guide/en/kibana/current/dashboard.html#embedding-dashboards.