Here is the full list of all 37 LEGO Harry Potter: Years 1-4 trophies - there are 18 bronze, 14 silver, 4 gold and 1 platinum.
Collect all the bronze, silver and gold trophies
Complete the Quidditch level within five minutes
Defeat the mountain troll, using Quirrell
Hide in a barrel as Snape
Defeat Harry (as Voldemort), in the Graveyard
Defeat Quirrell in the Quirrell boss fight, using Voldemort
Complete the game to 100%
Buy every version of Voldemort (Quirrell/Tom Riddle/Voldemort)
Collect all of the Gold Bricks
Buy the entire Gryffindor Quidditch team from Book One
Rescue all students in peril
Buy the entire Weasley family
Unlock all Animagi
Scare 20 students using a ghost character
Collect all of the Red Bricks
Destroy 50 Devil's Snare plants
Collect all of the house crests in Year 1
Collect all of the house crests in Year 4
Collect all of the house crests in Year 3
Defeat 10 enemies with a Muggle character
Collect all of the house crests in Year 2
Defeat 20 enemies with Fang
Get True Wizard in every level
Complete the Story Levels in all four years
Complete Year Four Story Levels
Use the Time-Turner
Complete All Story Lessons
Complete Year Three Story Levels
Complete Year Two Story Levels
Defeat 20 spiders with Ron
Freeze 20 characters using Glacius
Collect 500 ghost studs
Complete all the LEGO Builder tutorial levels.
Complete Year One Story Levels
Turn the sound down to 0 and music off in the options menu whilst in the library
Collect 10 million studs in one level
Knock over ten characters using a ride-able object