Concept: The question tests the ability to solve Word Problems
The first shelf can have any 1 of the 10 trophies.
The 2nd shelf can have any one of the remaining 9 trophies.
The 3rd shelf - any of the remaining 8 and the 4th shelf any of the remaining 4.
Total Possible ways = 10 * 9 * 8 * 7 = 5040