Here's how you can get the crest in Lego Harry Potter: Years 1-4:
1. Explore the levels: The crest pieces are hidden in various levels, often in secret areas or behind breakable objects.
2. Look for glowing spots: Pay attention to glowing spots on the ground, walls, or ceilings. These usually indicate hidden items like the crest pieces.
3. Use spells and abilities: You'll need to use different spells and abilities to reach certain areas or break objects.
4. Complete side quests: Some side quests might reward you with crest pieces.
Remember: The Slytherin crest, along with the other house crests, is a reward for finding all the missing pieces, not a specific part of the crest you can collect.