Jobs!
Play the song first, and then give them the lyrics (with transliteration if you have some really low level students). Do a listen and repeat with the sentences or words they're going to struggle with. You may have to sing a couple of lines so the kids can recognize where the sentence they're saying fits in the song, but they're 4th graders - they don't care if you suck. If it's a tough song, or they're struggling, stop every few lines or so and practice with them.I'm not sure which song you mean, so if I'm wrong, I'm really sorry. But if by "the cups song" you mean the one Anna Kendrick sings in Pitch Perfect, I honestly think that's going to be way too hard for them. Even if you have high level kids, it's really fast and has lots of expressions they won't know. (Also, it mentions whisky, so you may want to run it by your co-teacher first, since he or she will probably end up fielding questions from the kids about what the song means.)Personally, I normally stick with ESL songs from YouTube for fourth graders. We sang "If You're Happy and You Know It" last month, and the kids had a good time with it. If you really want to do a pop song, I think you're better off with something like "Lemon Tree" by Fool's Garden. I know everyone uses it, but there's a reason - it's a much more doable pace, and they'll be familiar with a lot of the vocabulary.
Agree with some of the other posters about it being a little too tricky for 4th grade. I just did it with 6th grade and while they loved it and the lesson went really well, they struggled with some of the lyrics and even after two full classes of practice they still mumbled through the "mountains, rivers, sights to give you shivers" part, they just couldn't get the words out.The 4th graders in my school are currently learning Bingo was his name-o complete with all the clapping and actions and are deliriously entertained by it.