Looks like a pretty good, inexpensive solution (with your existing grills) if you're going with 4" coaxials.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=230360080742
Based on the design of the grill, it looks like a separate mid & tweeter should work, but I believe there's a structural member in the way where the tweeter would go that makes this very difficult if not impossible.
Gary