Ive render it at 3900 by 1500px which took my about 6hrs, my computer (no-beefy) was sweating begging me to stop

.
When Ive import it to ps the dimensions read 50in

, so I reduce the size to the final size at 9 x 4.5in with 400dpi, still with the same pixels of 3900 by 1500px. It has multi-pass, which my current version of carrara5P doesn't have.
thank for the imput guys, and Rickei--I'll keep that in mind, I just don't feel like re-rendering it again.