Here is the full list of all 37 LEGO Harry Potter: Years 1-4 trophies - there are 20 bronze, 12 silver, 4 gold and 1 platinum.
Collect all the bronze, silver and gold trophies
Destroy 50 Devil's Snare plants
Defeat Harry (as Voldemort), in the Graveyard
Defeat Quirrell in the Quirrell boss fight, using Voldemort
Defeat 10 enemies with a Muggle character
Defeat 20 spiders with Ron
Defeat the mountain troll, using Quirrell
Hide in a barrel as Snape
Complete the Quidditch level within five minutes
Complete the game to 100%
Buy every version of Voldemort (Quirrell/Tom Riddle/Voldemort)
Collect all of the Gold Bricks
Unlock all Animagi
Buy the entire Weasley family
Buy the entire Gryffindor Quidditch team from Book One
Rescue all students in peril
Collect all of the Red Bricks
Complete all the LEGO Builder tutorial levels.
Scare 20 students using a ghost character
Use the Time-Turner
Collect all of the house crests in Year 4
Collect all of the house crests in Year 3
Collect all of the house crests in Year 2
Knock over ten characters using a ride-able object
Collect all of the house crests in Year 1
Get True Wizard in every level
Complete the Story Levels in all four years
Complete Year Four Story Levels
Freeze 20 characters using Glacius
Complete All Story Lessons
Complete Year Three Story Levels
Complete Year Two Story Levels
Complete Year One Story Levels
Defeat 20 enemies with Fang
Turn the sound and music down to 0 in the options menu whilst in the library
Collect 500 ghost studs
Collect 10 million studs in one level