" what are the different things/issues that come up if switch from 6 metal layer process to 4 metal layer process and vice versa for the same design.. Issues interms of timing, congestion,area..Please add issues that you think are critical"..
As a very preliminary answer I would say you must consider the routing congestion you'll have and the AREA since all the lines you were having in TWO metal layers will need to be distributed between the available 4, in other words, you'll need a much bigger routing area in those leyers...
I do not see why timing could be an issue, at least at first sight it shouldn't be affected.
area, from 6 to 4, i am assuming placement area. any macros, that take up 5/6? if not, i don't see a problem here. routing, from 6 to 4, less rounting resources. may also cause timing issue due to detouring of nets. power mesh, only on 4? IR drop may become an issue.
i cannot think of anything else. but i am sure, i am missing alot.