If repeats are allowed, then it is 10^4 = 10,000.
If repeats aren't allowed then it is 10C4 = 210.
If you need help understanding math so you can solve these problems yourself, then one on one online tutoring is the answer ($30/hr). If you need faster solutions with guaranteed detailed answers, then go with personal problem solving ($3.50-$5.50 per problem). Contact me at fcabanski@hotmail.com