From the one days experience I have of Mumbai ( ), we did a lot on that day and felt one more day would have been good. We did not see Elephanta Island. I think 4 may well be too much. The one day we had was an entire day as we had arrived the night before and stayed a further night. 2 or 3 clear days would let you see a lot.
However, hopefully those with far more experience of Mumbai will advise
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I'll create a link to this from the Goa forum to see if that helps

Thanks Fiona, we're thinking 3 nights now, but can't decide!

I love Mumbai, it is not as touristy as Goa, but it has lots of things to see and do, There is the Elephanta Island, which is a must see and do, Crawford Market, Thieves Market, Museums, and you have more of the true feeling of India, nothing is whitwashed just for the tourists, Also everything except alchohol and the price of hotel rooms, is cheaper in Mumbai, You think Goa is cheap well you should see the prices in Mumbai, I always try to leave my buying until I get there, because there is nothing more infuriating than finding the souvenier you bought in Goa is selling for a third of the price in Mumbai.

It is a fascinating City.

Stayed for 4 nights in April, it's a fabulous city with plenty to do and many good and inexpensive restaurants.

Read this thread, my review is included on the last page.

http://www.holidaytruths.co.uk/viewtopic.php?t=92711&postdays=0&postorder=asc&start=80