Bankwest, Heritage and a few lower tier lenders will do 4 dwellings on one title. You can also do 4 under a commercial loan if you want but this may require a lower LVR
I know Heritage have become more restrictive and lowered their LVRs for 4.
You could do a 3 unit development then you have a lot more finance options - 4 really restricts the number of banks you can use.
I think you guys want a challenge but you need to work out if you have the temperament for it. It does take a lot of patience and stress x 1000 what you have...