Drum scans are the shit. My lab here does them
http://www.robinimaging.com/ Indeed you can get photo paper that large. I believe you order it on rolls. However, you'd need to have a 4x5 enlarger with a color head. Turns out enlargers of that caliber are
extremely expensive. I have found used ones with color heads for around $1500.
You would also need to get a vacuum easel to hold the paper flat, which can be expensive (but can be built easily and cheaply).
On optical prints:
With the latest digital inkjet printers, C prints are somewhat a thing of the past and typically only done for purists. A print with a high end plotter or other inkjet device, on good paper, with dye-based inks and a high quality drum scan will easily produce prints that are superior to C prints.
The real advantage to C prints is archival quality, but were talking about color so you mind as well toss the idea of an archival color print out the window.