Eric, I made several errors on my last post which has probably caused confusion for you and others following along. The test I did was actually 4800x7200 at 300 ppi for a 16x24 print. I believe I called it 4200x7200. That was a typo.
But more importantly I switched sizes on you in midstream. I am considering a 24x36, not a 16x24. Somehow, yesterday when I exported the two example images, I got 16x24 in my head and that was how I exported the enlarged version. Not sure what I was thinking. 7200x10,800 is correct for a 24x36 at 300 ppi. Sorry for the confusion. I will try an exported image from LR4 at 7200x10800 and see how that looks zoomed in to actual print size as you suggested.
Thank you for all your insight and for taking time to help with my questions.