I just purchased some new shingles for my roof. I just noticed that they're 4 years old. Is this considered old for new shingles?

The shingles were manufactured 4 years ago. You could return them for some newer manufacture date but since they were only stored somewhere it should not effect their performance on your roof,so long as they are not brittle, faded out or broken. If in doubt, return them and get an exchange. This is just new old stock they sold you.