Here is the full list of all 36 LEGO Harry Potter: Years 1-4 achievements worth 1,000 gamerscore.
Buy the entire Gryffindor Quidditch team from Book One (Single Player Only)
Turn the sound and music down to 0 in the options menu whilst in the library
Unlock all Animagi (Single Player Only)
Buy the entire Weasley family (Single Player Only)
Collect all of the house crests in Year 4 (Single Player Only)
Hide in a barrel as Snape
Collect all of the house crests in Year 3 (Single Player Only)
Defeat 10 enemies with a Muggle character
Complete all the LEGO Builder tutorial levels (Single Player Only)
Defeat 20 spiders with Ron
Collect all of the house crests in Year 2 (Single Player Only)
Collect all of the house crests in Year 1 (Single Player Only)
Defeat 20 enemies with Fang
Complete Year Four Story Levels
Complete the Story Levels in all four years
Complete the Quidditch level within five minutes
Get True Wizard in every level (Single Player Only)
Complete Year Three Story Levels
Complete All Story Lessons (Single Player Only)
Use the Time-Turner
Knock over ten characters using a ride-able object
Complete Year Two Story Levels
Complete Year One Story Levels
Collect 500 ghost studs
Collect 10 million studs in one level (Single Player Only)
Collect all of the Red Bricks (Single Player Only)
Collect all of the Gold Bricks (Single Player Only)
Rescue all students in peril (Single Player Only)
Complete the game to 100% (Single Player Only)
Buy every version of Voldemort (Quirrell/Tom Riddle/Voldemort) (Single Player Only)
Defeat Quirrell in the Quirrell boss fight, using Voldemort
Freeze 20 characters using Glacius
Scare 20 students using a ghost character
Defeat the mountain troll, using Quirrell
Defeat Harry (as Voldemort), in the Graveyard
Destroy 50 Devil's Snare plants