I installed 4.0.1 and then when upgrade available i did it to 4.0.2.
Now I am not able to see GUI for template editor as attached below…
Any idea???
That is working. There aren't any templates at that location. Go down into addons/affiliates/blocks for example.
If you're looking for the GUI interface you should be looking at the Themes and Layouts on the Design menu.
in the Themes I have made some customization and named it xyz theme. Now when i click on the customize link it takes me to the my website rather giving me customization for color and all at the left side… 
Is there any setting missing for this or it is purely bug…
Look on the left hand side of your webpage. You might have the theme editor minimised. Clicking on the arrow will bring it back. If that doesn't work then try refreshing the page in your browser.
No.
It is not… there is no arrow.
Initially it shows url “http://.com/admin.php?dispatch=customization.update_mode&type=theme_editor&status=enable”
but suddenly it goes to and url is <mywebsite.com>/index.php
I had the same problem, upgraded from 4.0.1 to 4.0.2 and the theme editor no longer worked, ie. it no longer appeared on the frontend when I clicked the Customise button in the admin. In the end, I re-installed 4.0.2 from scratch and re-applied the changes I had made in 4.0.1. I do recall reading about this somewhere on the forum/bug tracker where other users had encountered the same problem and it was a simple fix (something about company_id IIRC) in the database which resolved it, but I can't seem to find the thread again.
Any updates on this? CS-Cart ppl takes too much time to resolve??
Did you raise a support ticket for it? Mine always get sorted within 48 hours.