Where you find the deep water you find the big fish. Dont rig a whole 5kg fish, just half it and use the other half for fillets. Fish wiith bait to the size of the fish you know are in the dam or could be there. If i know there is 30kg plus fish 4kg bait is not that big, that is just over 10% of its body weight and its mouth will easily take the fish. Ive seen some of hennies private photos of a 3kg carp in a 25kg fish, but it will take you time to buil the confidence in what size baits will actually work. Its one of my favorite sayings, fish are lazy, they want to spend the least amount of energy for the moet amount of gain. If you put out a 2kg fish the fish that eats it will than not have to hunt for awhile and thus safe and censerve energy, thats what it wants to do. So a big bait is a must, a large cat will pick up a small bait if it happens to be right next to him. But more often than not the bigger the bait the bigger the fish. Also if the bait is large it wont get taken by small fish as it wont fit in its mouth. You than give yourself a great chance of having the bait undisturbed in the right spot. Out of intrest, the next time you go fishing and you cath anything up to 15kg have a look at what you can actually fit inside the mouth, you will be surprised.
In rigging the bait, skuimkop has posted in one of the post with a diagram, on a bait that big i would rather use a 2 hook trace than the rig you used for the half. But again you could teach me something and prove that it works, my rigs would be alot easier if it did. On my baits i cut away gill plate, spray dead red inside the gills, leave it for 5 min than give it a quick spray as i take it out. I would slash it as you mentioned, but also have a small cut into the stomach cavity. Let the strong scent from the guts get into the water. Use as much natural scent as possible and use the dead red as an attracter, when you first use it, dont go overboard, a couple sprays is enough to last awhile.
How quick where you striking on the plattie and how where you stiking? Could be the fish had it clamped between the 2 bonny plates in its mouth and was just holding on, as with all live bait i let my fish swim for alonger time than a dead bait to make sure it swallows the bait properly.