I've only been to Bombay twice, once as a small kid and recently for business, so I'm not the best qualified.

Though I can state with some assurance that a quick search on this forum would help, as would a glance at wikitravel (just as a base, fyi there's a two day itinerary on the page). Personally, 4 days in Mumbai might be a big much since its more of a commercial city rather than historical/cultural. That being said, I have yet to really tour it, so I could be wrong.