Forever in Blue: the fourth summer of the sisterhood by AnnBrashares
This is supposed to be the last installment in the Travelling Pants series, although the ending does seem to leave open the option for a sequel or a spin-off.
In this book, the four friends are all in college in various parts of the country. They've been unable to actually all get together for some time, and have been relying on emails, telephone calls and the Pants to keep them connected.
After this first year of college, they all face more "grown-up" challenges: a pregnancy scare, a malicious roommate, and how to know when you're really in love. One thing that they all learn: although the closeness and generosity of their friendship has made them a bit naive about relationships in "the real world", they are all confident and capable young women, even without the Pants.