There are four children in my class who might 'do a runner' at any time during the day. Only one has a 1-1 worker. It's increasingly common in young children. More and more are starting school unwilling or unable to follow instructions.
Even if the child has SN, even if they have full funding, the parent still needs to work with the school to help implement an action plan.
It is possible to restrain children, if the staff have been specifically trained for this, but it would be a last resort. OP's son is only four but as the children grow bigger, the problems, if not addressed, become more and more difficult to manage.
This is why so many children go 'off the rails' during their teenage years. They are too old to be confined to time out. They have learned that, actually, there is nothing adults can do to stop them doing whatever the hell they like.
Once they become adults, they can of course be arrested and forced into 'time out' via prison, but no-one would want that for any child. It's so important to face up to the problem now, whilst he is young enough to learn and change.
Saying that school should ignore him, that works best, is not a strategy suitable for school or, indeed, for the parent, long term. He needs to be taught to comply and that means parental involvement, if it is to be successful.
Yes, the school have a duty to educate him and they will do that as best they can. But in seven short years he will be gone from that school. The parent is best placed to help him, support him, teach him and guide him through his young years so that he can grow to be a well balanced man who understands how he fits into the world and his responsibilities towards others.