L.R.
You don't say what form of "school" she is in. If it's a preschool, consider whether she is going for too many hours each day and her concentration flags because she is simply too tired. See if you can have her in half-day preschool if it's full-day, or have her in preschool 3 days a week instead of five, for instance.
Also consider whether this preschool is the right fit. If they are trying to be too "academic" with children this age, please reconsider and think about finding another school. Preschool is about socialization; learning to follow directions from an adult who is not mom or dad; learning to stop one activity and transition to a new one without upset; learning to share and get along with other kids. It is not about how many worksheets kids can do or how many projects they can complete. If the preschool is focused more on academics than on socialization and learnign through play, it may be doing her a disservice. You live an an area that is very competitive and very academically driven (I know -- I live in the same area!). So do a reality check and ensure you're not letting other parents' attitudes about what kids are "supposed to know" by any certain age get under your skin.
Ask yourself: Are the teachers' expectations, and your own, truly realistic for a four-year-old? Or are you expecting behavior she is just not developmentally ready for at four because no four-year-old really is? Have you looked up some good books on child development to check out where she really should be right now?
If you feel that expectations and the school's activities are realistic for her age and stage, then get details from the teachers and as others noted, ensure she is being compared to four-year-old peers, not to the behaviors one would expect of older kids.
You also mention that she gets bored and wants to stop in after-school activities. That is completey typical at her age and not a sign of anything wrong with her attention span. Ensure that her activities are ones SHE really wants to do; if she says she does and the attention still flags, check:
Is the activity/class session too long? (30 miinutes to an hour max at this age.)
Is the activity after school when she is already tired out and maybe hungry? (Consider whether she is over-scheduled and/or needs a protein snack and down time before going from school into another activity. If she has more than one after-school activity at this age, consider dropping down to just one. Kids need down time.)
Is the activity too repetitive so it's the same week after week and is boring her? (Sit in and really watch the teacher.)
Is the session of the activity or class too long? (Four is young to have an activity every single week for the entire school year; look for sessions of six, eight or 10 weeks that end after that period.)