Think about it like this: o is a marble and | are dividers into 4 jars. For example we could have o | ooo | o | oo, which is 1, 3, 1, 2 in the four jars.
There are 7+3 things total, which is 10. There are 3 dividers, so the number of orders of this "string" of o's and |'s is 10 choose 3 = 120.
You are very welcome!