Can't she just take the baby? Personally, my nursing relationship with my child is worth far more than a trip with "the girls". So maybe she doesn't go to Pleasure Island or Jelloyrolls while the rest of you do. Honestly, one of my best trips was with my 4 month old DD (her first). She was very easy to travel with and slept in her stroller much of the time.
I agree. More than that, she will have to figure out whether or not pumping will work for her. Some mothers can pump a lot, other mothers can't. At four months old, my son was nursing every two hours. And by every two hours, I mean I had 90 minutes in between nursing sessions, from beginning to beginning.
In addition, it can often take up thirty minutes to pump. So, pumping for thirty minutes and then having to pump for another thirty minutes an hour and a half later can certainly get in the way.
If she chooses to not pump as frequently as her baby eats, she will need to anticipate a drop in milk supply by the time she gets home to her child. She needs to anticipate that she will be taking a risk to her milk supply and her nursing relationship to leave for four days.
That being said, there are women who have done it with planning and done it successfully. But, she should simply be prepared that she is taking the risk and whether or not she is comfortable with the risk of jepordizing nursing for a girls vacation. Does she even know if he'll take a bottle? What happens if she doesn't have enough milk stored up, will he even take formula? I know many exclusively breastfed babies that will not drink formula because of the significant taste difference.
Personally, four months is well younger than I would ever consider leaving my children by choice. And she should also seriously consider whether or not she is o.k. with not seeing her new baby for that long. It's one thing to fantasize being away from your baby, and it's a totally different experience to actually *be* away from your baby. A friend of mine was adamant they were going away for a long weekend for their anniversary and left their six month old with the grandparents. After 36 hours away from her son, they ended up back home early because she vastly underestimated how much she was going to miss him.