This thread is pure Mumsnet bonkers!
People do massively underestimate 4 Yr olds. They are not babies or even toddlers. They are school age children. Unless there are SEN, it is not unreasonable to expect them to understand what is and is not theirs and that they need to not touch, or treat with respect, the things that don't belong to them. They should be able to go into a cloakroom and get their own coat, for example, rather than just taking someone else's that they like the look of.
They should also be capable of doing some things unsupervised. For example, in school they will go to the toilet and wash their hands independently. They could go and stick their head in the toilet and flush but it is reasonable to expect that they won't do that.
Of course young children make mistakes sometimes. They are still learning. But they will not learn if they are not held responsible for their behaviour and have consequences when they choose to do something that they should not have done.
It is completely age appropriate for a 4 yr old to watch TV unsupervised. It is also completely appropriate to expect them not to deliberately open someone else's presents. He got it wrong this time. It is reasonable to be cross and upset about what he did and for him to have consequences for that. My preferred consequences would be natural consequences. He should understand what he did was wrong, that it has upset people, and he should be sorry for that and apologise. He should help rewrap presents. Perhaps replace damaged things with something of his own or come to the shops and choose a replacement for his sister while he gets nothing.
I definitely would not shrug it off as him being too little to understand. He will learn nothing from that.
Some of these answers do go some way to explain why some children in school think it's perfectly OK for them to trash stuff though. Unfortunately, it is then the other children having to go without that really suffer the consequences. Near me a bunch of teens vandalised a children's playground so thoroughly and frequently that it ended up being removed altogether. That's where lack of consequences leads.