4 hours is cutting it close but if you want to chance it, take the airport express to Sentral and switch to the Putra line and head towards KLCC. You will emerge directly below the Petronas Twin Towers. If you haven't seen the Towers before, this would be the #1 thing to do by day or night for any tourist to KL. If you do only one thing in KL, this should be it. After that, you can sample some local cuisine (Malay, Indian, Chinese, etc.) at the food court in the Suria KLCC mall located directly beneath the Petronas Twin Towers. The food is (dirt) cheap and pretty good. If you prefer a restaurant setting (although I doubt you'll have time for it), Madam Kwan's on the top floor of the mall serves pretty good local cuisine Nyonya-style.

You definitely don't want to linger too long in the city. An hour at the most. If immigration lines are long entering Malaysia, I would just turn around and go to the lounge.

Your alternative is to take the airport express to Putrajaya, the new capital city. It's much closer to the airport than KL. Once there, you should be able to cab to the local mall (called Alamanda) and sample local cuisine at the food court. I've never been so I can't vouch for it. There are also some interesting sights in Putrajaya. Ask the cabbie to take you to Dataran Putra (or Putra Square). Might not be much at night though but worth a look for the impressive Islamic architecture.