You can check Wikipedia for the dates.
Volume 4 is the last one. Season 8 ends with the Lat Gleaming arc and the Riley one-shot story, which are both in volume 4.
After that the comics go on to "Buffy Season 9 and also "Angel & Faith" as the two main titles. They will both be 25 issues each, the individual issues finishing summer 2013. I do not know if they ill do more library additions because they are so much shorter then Season 8.
There is also two mini series that are 5 issues each. "Spike: A Dark Place" finished last month. "Willow: Wonderland" finishes in March.
You might just want to get paperbacks of them, the order is:
S9: Freefall/A & F: Live Through This
S9: On Your Own-Apart (of Me)"/A & F Daddy Issues
S9: Guarded -Billy the Vampire Slayer/Mini-Spike: A Dark Place/ A & F: Family Reunion
S9: Welcome to the Team/Mini-Willow: Wonderland/A & F: Death and Consequences
A & F's last arc is called "What You Want, Not What You Need" starting in monthly issues April 2013. I don't know the name of the last Season 9 arc yet.
There was going to be a Drusilla mini-series set after Daddy Issues, written by Juliet Landau, but it has been delayed by scheduling issues. I don't know when it will come out as of now.
Freefall, On Your Own, Live Through This and Daddy Issues are all out in paperbacks at big chain bookstores and Amazon now.
Family Reunion paperback is out in late April I think, Guarded paperback is out in early May I think, Spike: A Dark Place paperback is out in early July I think.
A "Buffy Season 10" is planned, no word on what will happen to Angel yet.