What are my chances of clearing a KrisFlyer waitlist flight award?

looking to book 4 J return tickets next year around march out bound and April return., melb to Milan return. 

The waitlist is just a way for Singapore to manage the award space on any given flight.  In essence, you should look at it as if space is unavailable, but that if space is made available you will be offered the chance to grab it before the general public.

I have only waitlisted once, Suites class HKG - SYD for one ticket.  My waitlist cleared about 6 days before the departure date, but your mileage may vary.

One thing I have read is that if you waitlist two tickets, you will only be pulled off the waitlist if two spaces open up.  This means that if Singapore opens up one seat, that seat will go to the first person on the waitlist who has booked one ticket. 

In your situation, you are hoping that singapore opens up two concurrent seats on 4 flights (two returns), seats which are not yet available.  I wouldn't count on the waitlist clearing for every seat you've waitlisted for.