I matched at Pitt for this coming year, but I didn't do an away rotation there so I can't speak specifically as to which place is better to do a rotation as a 4th year. I also don't know if Pitt offers the option of doing 2 weeks @ UPMC and 2 at Mercy, but my home institution had a similar setup and I found that worked well for me. Working at the two sites would give you face time with the most attendings as possible, experience EM in two different settings, and scout out the two places where, in aggregate, you'd spend 90% of your time during residency. In terms of face time with the PD, I think Dr. Wolfson (can't bring myself to call him Abby just yet) only works @ UPMC though - however, I wouldn't worry about that. You'll get a chance to meet him and by doing well in the rotation, you'll help make yourself known.
Again, I don't know what Pitt does specifically for 4th years, but I would look into doing a combined Mercy and UPMC if it's offered. Ask me in 4 months and mabye my answer will change