Well, it really depends on your "group" and how you decide to run things. Most groups are 18 students w/equal # of D3s and D4s. In my group D4s get their own chair for every session. The D3's have a "wait list" they rotate on, but generally they get a chair because of cancelations or seniors not being in clinic (interviews, externships, etc.). Last year I only sent 1 patient home because I could not get a chair in the first half hour and that was because people were screening for the NERBs. So, from my standpoint, it is a little bit of a pain as a D3 but it is great when your a D4 (when it counts). I will admit that some of my classmates have not been as lucky though.
Last thing I will mention- Although penn is not perfect, and chairs are sometimes an issue, we have a surplus of patients (thats why our requirements can be so high). Make sure you end up at a place where you will be busy ... I couldn't imagine having to do dentiform prepps to graduate because I haven't had enough clinical experience.