OK, the best way to extend activity time when your LO is getting tired is to take them up to their room and play with them on the floor - nice quiet activities, but ones that have them interact with you so they don't fall asleep on you
Books, stuffed animals, stacking cups, rings, things that don't light up or make lots of noise. That way you're sort of beginning your wind down and keeping them up longer at the same time.
Jimena - it's also possible that your LO needs something like a 3.5 hour EASY at this point - lots of LOs don't transition right from 3 to 4 with no trouble. I think we did a 3.5 hour until about 5 months when she was able to go to the full 4 hours.
Jenna's mom - if the distracting works, I would keep donig it. And as for teeth, it can last for quite awhile -- you can try giving her a cold/slightly frozen washcloth to chew on or a cold teether to see if that helps. I hear you about not wanting to give Tylenol all the time --- I always try to reserve it just for nights and have her chew on things during the day -- if it's any consolation, they do get (sort of) used to after awhile.