hi allison; it sounds like your baby needs an extra nap.
i went thru this w my first child; i didn't realize that after 1 pm he was getting very over tired, this started when he was about 5 or 6 months old. he would have a morning nap, at 9 or 10 a.m., for about 2 hours, and then start to get tired about 1 or 2 pm; by 4 he was really miserable. what i started to do was put him down for a second nap about 1 or 2 pm, for another 2 hours or so. then he would have a very nice after noon and i would start his night time bed process about 630.
babies need to sleep about 12-15 hours per day and are usually not able to sustain being awake for more than 3 hours at a time. 4-5 hours of being awake for a less-than-1-yr baby is really pushing it. make sure you are putting her to bed early enough, too. not later than 630 to start the bed time ritual. try reading for some ideas about bed time routine "No Cry Sleep Solution".
some kids will start to give up thier naps between 1 and 2; some will give up one nap first and then the second one, some will stay awake longer in the morning and combine the naps for a long afternoon nap, that's what my son did. the thing is not to count on getting it "right" because it will change again in a few months.
my daughter is 15 months and sleeps from 9 or so for 2 hours in the morning, and then again from 2 or 3 for one to two hours. she has been pretty consistent with this but not 100%. they are all different.
good luck!
J.