LEGO Harry Potter: Years 1-4 is the first installment of the Harry Potter series in LEGO form. Spanning the first four years of the Harry Potter franchise, players take control of a wide variety of characters and relive famous moments from the first four books; Sorcerer's Stone, Chamber of Secrets, Prisoner of Azkaban, and Goblet of Fire. Almost all of the game takes place inside Hogwarts.

The Story mode is broken up into four years with six chapter per year. When you first play story mode you are required to play as certain characters that are part of that story and you cannot choose any character from the entire series. But once you beat that chapter a Free Mode is unlocked, allowing you to pick any unlocked character and replay that story. All story chapters and years can be accessed in The Leaky Cauldron which acts as the central hub in the game. The entire environment allows for continuous play and most places can be revisited at any time. Many of the characters' interactions change however based on the time of year, or when certain characters are introduced such as the Dementors or Gilderoy Lockhart.

Outside of The Leaky Cauldron is Diagon Alley. In Diagon Alley you can head to other stores to obtain items and supplies. At Madam Malkin's Robes you can spend all of your hard earned studs (the currency) to buy new characters. At Eeylops Eye Emporium you can head upstairs and buy Red Bricks which contain cheat codes of sorts to use. And at Wiseacre's Wizarding Supplies you can buy fun new spells to cast on anybody you like. Casting spells is an important part of the gameplay and is accessed by a spell wheel in which you can choose many ranging from Wingardium Leviosa to the dreaded Aveda Kedavra (only the three versions of Voldemort can use this).

LEGO Harry Potter adds to the replay value by having a bunch of collectibles for you to obtain. If you want to get 100% completion you're going to have to do things such as find and purchase all 167 characters, rescue 50 students in peril, find 200 gold bricks, and much more. Hogwarts school is open to you at all time where you can search the many rooms and floors for all these collectibles. You will have to wait though if you try to get everything early because many characters have their own special abilities pertaining to them.

There is no online multiplayer in LEGO Harry Potter but there is co-op. At any time another person can join you in the action, played on a single screen. Similar to LEGO Indiana Jones 2: The Adventure Continues, when one character goes off screen, it is split automatically and the game goes on that way until they are close again.