How does the four card limit work?
Other than basic lands (Island, Forest, Plains, Swamp, Mountain, and Waste) you are only allowed to have four cards in your deck with a given name. (Barring a few exceptions where the card itself says otherwise, or the card is restricted in the format you're playing.)

As an example, you are allowed to have four copies of Plague Rats, and four copies of Ravenous Rats, you can also have any number of Relentless Rats, as the card has specific text allowing "any number" which overrides the four card limit.

In short, you are allowed any number of distinct card names, but only four instances of any given name.