You have to figure out why she is barking before you try to cure the problem.
Sometimes puppies get the nighttime "crazies" and this can be fueled by the hot weather. The hot weather during the day (I'll go ahead and assume your area is hot!) makes them feel sluggish so when it gets cooler at night they are suddenly raring to go. Try tiring her out before bed and see if that helps. A 4 month old puppy following the 5 minute exercise rule needs at least 20 minutes of activity once but preferably twice a day.
I'm kind of thinking that it may be the heat as Trickster mentioned. It has been very hot here in the bay area, 100+ for the last week at least. Today is supposed to be just 100, but the humidity is going to be 35% which is really high for us.
Lexie is fine during the day, she crates without a problem and sleeps while I'm at work (my mom is here so I know that she's quiet). In the evening it's still pretty warm (85 @ 8:00 p.m.) so we wait until it cools off and then we take her for a walk, then we usually practice retrieving and training with her. So she's getting her exercise.
She doesn't like being away from me in the evening and if I crate her before I go to bed and then try to have some quite time to watch TV, read a mag/book, and/or get some computer work done she barks. If I go to sleep when she goes to sleep then she doesn't bark. She wasn't doing this before, it really just started over the last week. I'm home in the morning and she doesn't do it in the morning when I shower, etc.
After Rob posted this question I was looking in Lexie's mouth and she's teething. I can see new teeth, so I'm thinking this could be causing some of the barking too. I'm giving her frozen wash clothes and other frozen chews to help her with the teething pain.
I did a search of the forum last night and found some threads on barking. I read the suggestions on putting the put in the bathroom, lemon juice, and tapping on her chin. We're going to work on teaching quiet over the next couple of weeks.