First: Don't panic. It won't help you find a working solution and it won't make you feel better, either.
Second: Be grateful. Most of these tricky questions have already been answered by the brave women who came before us - those who pioneered the concept of "Women in the workplace" are also responsible for discovering innovative childcare solutions.
Three: Your options include (but are not limited to) the following:
1) KinderCare or a similar felxible drop off/pick-up child care facility.
2) Co-Abode: w w w . co-abode . com This website is a not for profit organization that helps single mothers find other single mothers to live with - the goal being shared living expenses as well as cooperative childcare options.
3) If you have a spare room in your home (or can make a spare room with some shifting around) then consider renting the room to a college student for childcare in exchange for rent. This would help a college student who is struggling with expenses and it provides you with 20 hours a week of FREE LIVE-IN childcare (live-in childcare can be a lifesaver if you are ever stuck in a traffic jam, or you are forced to work overtime or your child is sick at school, or in any kind of an emergency).
4) If your job provides enough financial freedom you might consider sponsering an Au Pair to live in with you. Usually they are cheaper than a live-out Nanny and they often do light housework while your child is in school. They are often contientious and caring people who would not have the opportunity to live and work in the U.S. without a sponser like yourself.
5) You might also consider placing an ad in the local paper for a Nanny-Share arangement with another family who also needs part-time childcare on an irregular basis. For example, they only want childcare in the morning 2 -3 days a week. You could use the Nanny in the afternoon (after school) until you get off from work. Again, you would be helping another family while helping yourself at the same time.
6) In many of the larger cities there are often childcare co-opts that you might be able to join. Since you get off of work at 4 every day of some of the weeks in a month you might be able to provide much needed "gap-sitting" (Afterschool until 6 or 7 pm) in exchange for the same until you get off at 10 pm. In this situation you might have to agree to either pay for some of the extra hours of childcare or commit to providing more than the standard amount of coop childcare - but it's still an option.
7) Don't forget that there are often volunteers at church (or whatever) who can offer a few hours of childcare a few days a week. Check your community senior citizen resources for more information.
WARNING: Be very careful about using a teenager to "babysit" for more than a few hours a week. Remember that teenagers are just really big KIDS and they are about as reliable as any other child would be. I have always thought that its unreasonable to expect a child to behave like an adult more than 2 -3 hours per week. No matter how much of the the financial burden would be reduced by hiring underage childcare - the savings is never going to be as valuable as the life of your child.