Is it possible to only volunteer for half a year or 3-4 months?

Yes, the Ecuador program has a summer opportunity that lasts 8 weeks. Learn more here: http://worldteach.org/programs/ecuador-summer/. There are other programs that a shorter in duration as well. To learn more about those, visit: http://worldteach.org/explore/.

No, the program is for a full year.

While I served quite some time ago, I believe the commitment is still 1 year. There was a summer program at one point so that could be an option depending on if the fit is right for you. Check their website. Quito has a fair amount of NGOs if you wanted to try to set up your own sort of assignment. You should be able to do internet searches, etc. to figure that out.