Here's some options, based on what I've found out or done myself since I arrived in Victoria earlier this year:
1. Wilson's Prom southern circuit (northern circuit is currently closed).
http://www.parkweb.vic.gov.au/1park_display.cfm?park=217,
http://www.gang-gang.net/nomad/greatoceanwalk/wp01.htm, including a side trip to South Point, the most southern tip of the continent. There are a number of variations, but a clockwise loop is the most common. I think the Refuge Cove campsite is the best of them, so I would try to work this one in to my itinerary. The southern circuit is on well defined trails with good campsites, but can be busy at holiday time.
2. Great Ocean Walk. In 3-4 nights you would probably only do part of it.
http://www.visitvictoria.com/displayobject.cfm/objectid.4AE3F158-9782-45D6-A7FC88F3C040AFAD/. I liked Johanna Beach and Ryan's Den campsites the best. The walk involves some fantastic scenery and trails, but also unfortunately involves quite a lot of walking on gravel roads and 4WD tracks. It's point to point, so you need to arrange transport
http://www.visitvictoria.com/displayobject.cfm/objectid.5B47D45C-0A61-46B7-98413F50F183B61D/, car shuttle, etc. Apparently the section from Shelly Beach to Blanket Bay is pretty muddy at the moment. You need to book a couple of weeks in advance due to the manual booking system - apparently they can't get funding to set up an online booking system.
3. Great Dividing Trail
http://gdt.org.au/Home. Could do any portion I guess, but I only have experience of walking the Lederderg section. Given the rain lately, is probably pretty wet and muddy in the gorge at the moment. Don't know how accessible the lower part of the gorge would be right now.
4. Grampians/Gariwerd.
http://www.parkweb.vic.gov.au/1park_display.cfm?park=109. Not so easy to put together 3-4 day walks until (if!) the new trail is built. But lots of options for day or overnight walks with a base camp at Halls Gap. Check out the Grampians walk booklet:
http://home.vicnet.net.au/~dvbc/grampians.pdf.
5. Bogong Circuit. A number of options for circuits - check out this recent trip report:
http://bushwalk.com/forum/viewtopic.php?f=46&t=5073.
6. Viking Razor Circuit. Best description is in John Chapman's book I believe
http://www.john.chapman.name/pub-aus.html. Here's some photos I found searching around
http://www.pbase.com/tlun/viking. Hoping to do this one myself this summer.
Bon courage!