For the record, this is my opinion, I cut/competed once so far so I am by no means an expert on the subject.
If I was you I'd cut it all the night/morning before. I am faily lean (10% fat I'd guess) and I was able to drop 5 lbs from taking a dump and sitting in a hottub for 30 minues. It took minimal effort, so I would guess that you could easily drop the 3-4kg the night before (Remember you lose a couple pounds overnight too.
After writing this I realize I didn't really answer your question, I will post anyway.