Rules 4 to 10 apply in any condition of visibility: in clear weather and in fog alike.
Rule 4: Application
Rules in this Section apply to any condition of visibility.
In plain words: Section I is Rules 4 to 10 (look-out, safe speed, risk of collision, action to avoid collision, narrow channels and traffic separation schemes). They apply at all times, whether or not you can see the other vessel.
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 4 on one page
Rule 4: Application
Rules in this Section apply to any condition of visibility.
In plain words: Section I is Rules 4 to 10 (look-out, safe speed, risk of collision, action to avoid collision, narrow channels and traffic separation schemes). They apply at all times, whether or not you can see the other vessel.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.