[user104658]

All I can say to the rest of it, is that there are ALWAYS reasons for a child being aggressive or acting up. Always. Just because they're not obvious or because no one can be bothered to properly figure out what the issue is, doesn't mean there isn't one.

Shouting, threatening, coercing, bribing, blackmailing - none of these are appropriate "smacking substitutes". They're just more examples of lazy parenting. You absolutely CAN reason with a child of 4+ and reach a mutual understanding, if you take the time to properly know them and understand them. The key word is MUTUAL understanding and respect. Not imposing your will on a child and having them be obedient. An understood and respected child will be understanding and respectful. Most parents are just "too busy" (or too lazy, or too stupid) to bother and go with the easy options like shouting, screaming, blackmailing, threatening and hitting.