All four are excellent.
Some advantages of each school:
Tufts:
As Mastadon pointed out, you can combine IR and ES. Also, Tufts is kind of a hybrid, combining some of the advantages of a LAC with some of the advantages of a university.
Georgetown:
In terms of program prestige, the School of Foreign Service is tops on your list and is probably the top IR/FS program in the US. Also, being in DC, there are going to be plenty of internship opportunities.
Wellesley:
Wellesley is the most prestigious college/university on your list. It is probably the prettiest campus of the four. And you are able to take classes at MIT.
Barnard:
Barnard is located in Manhattan, so you are privy to all the culture, entertainment, and awesome delis (and other types of food…) that NYC has to offer. You are also able to take classes at Columbia.
You cannot make a wrong choice as long as you follow your gut (and brain) and choose the best fit for you. You’ll have a fuller picture once the FA offers come in.