One important difference between IEnumerable and List (besides one being an interface and the other being a concrete class) is that IEnumerable is read-only and List is not.
So if you need the ability to make permanent changes of any kind to your collection (add & remove), you'll need List. If you just need to read, sort and/or filter your collection, IEnumerable is sufficient for that purpose.
So in your practical example, if you wanted to add the four strings one at a time, you'd need List. But if you were instantiating your collection all at once, you could use IEnumerable.
IEnumerable firstFourLettersOfAlphabet = new[]{"a","b","c","d"};
You could then use LINQ to filter or sort the list however you wanted.
