I have noticed that when articles of a law are mentioned in a text there is no definite article. For example, it is usually "Article 4" and not "the Article 4". Would some please explain me the rule? I am also wondering for chapters of a book. Should I write "Chapter 4", or "the Chapter 4". Thank you in advance!
Proper nouns don't need articles.
Would you really consider saying even "the Mr Jones", let alone "the The church of England"?
"Article 4" and "Chapter 4" are titles in the same way that Mr Jones or InNopi are titles and do not normally need articles.
Rather rarely, we might say something like "this Mr Smith is not the same as that Mr Smith and neither of them is the Mr Smith who punched you in the nose" but that's clearly contradistinction, which follows different rules.
"This Mr Smith" in that context is not at all the same as, for instance, "This is Mr Smith…"
Conventions and declarations are special cases. “The" is usually added before the name of a convention or declaration because although the title is complete in and of itself, it is never used by itself and would not fit the normal rules if it was. That sounds like nonsense but fine examples might be the Geneva Convention or the Hague Convention. Strictly, there are no such things.
Over more than 100 years, at least a dozen separate conventions contributed to a body of work commonly grouped together as “the Hague Convention”. Another series of agreements clearly plural and without “the” in any of the individual titles is normally known as “the Geneva Convention”. Since even dedicated students almost never look at the front page of the report of any convention, that is normally ignored.
For slightly different reasons, the names or titles of modern laws make this more clear. The reference is usually to “the Thing in Question Act” although the actual title is almost certain to be either “Thing in Question Act” with no article, or “A Thing in Question Act”.
This seems to be a legal convention, rather than a grammatical rule, because the names of laws and the titles of books should occupy the same places in grammar but often, they don’t. Either way, since grammatical rules rarely but legal conventions often put people in gaol, they deserve special consideration.
Canada has “An Act to amend the Citizenship Act and to make consequential amendments to another Act” - shall we contract that as “An Act to amend…”?
Even in court, parliament or law-school, that act will be termed “the Act to amend…” or contracted into “the Citizenship Amendment Act…” although both knowingly distort the original name. Further, most listeners will hear the phrase capitalised, as “The Act to amend…”