It is in the Encounter Building Guidlines for (2014) 5e and in the Name CR. A Cr 4 monster is a medium challenge for a 4th level party. The standard assumption is one Monster vs a Party.
Step 1. Expected Challenge Rating
Pick the expected challenge rating (CR) for your monster. Knowing the monster’s expected challenge rating will help you figure out the monster’s proficiency bonus and other important combat statistics. Don’t worry about getting the challenge rating exactly right; you can make adjustments in later steps.
A single monster with a challenge rating equal to the adventurers’ level is, by itself, a fair challenge for a group of four characters. If the monster is meant to be fought in pairs or groups, its expected challenge rating should be lower than the party’s level.
Don’t fall into the trap of thinking that your monster must have a challenge rating equal to the level of the characters to be a worthy challenge. Keep in mind that monsters with a lower challenge rating can be a threat to higher-level characters when encountered in groups.
It's not saying "the standard encounter will be a party of 4 PCs, who will fight 1 monster." It's just explaining how CR works.