Generating all 24 permutations would work, but you don't have to do that. Your validation for the brute force method would require you to validate each digit, and you may as well build up the permutation as you validate each digit. Start with big numbers.
- sort numbers in high -> low
iterate over each digit [i] and search through the list until you find the highest number that fits the requirements.
[0] must be <=2
[1] must be <=3 if [0]==2
[2] must be <=5
[3] can be anything
Remove that number from the list, and place it in position [i]
repeat for each digit
Each of those conditions could be expressed as lambda function in a list, making it easy to separate the search loop from the conditions.
