You might choose to go to graduate school after getting a four-year degree for one of two reasons:

You want advanced knowledge that can be used in your career field (like getting a master's in business to earn a higher starting salary).
Your career requires skills or knowledge not available in an undergraduate program (like becoming a psychologist).

Whichever the case, remember that:

College costs (and your potential debt level) will continue to build up.
You likely will not be earning a full-time salary and attending school full-time.
Graduate school will probably be more demanding in academics and practical experiences than your undergraduate program.

Learn more about success in grad school in the ICAN Graduate Student Success Guide (PDF).