There is the odd factory 4 speed C body wagon. Very rare and technically not available from the factory, but some did get made. I know and have pictures somewhere of a 66 Fury III 440 4-speed wagon in white w/a blue interior... special order Arizona car IIRC...
What year and make/model is yours?
If it is a 69 and up it will be a true custom... there were only 3 speed column shift manual trans cars made for those years, and while some of those parts will function in a 4 speed conversion (like pedal and Z bar assemblies), you'll one of only a few pioneers who have attempted such a conversion.
C body 4 speeds were made from 65-68 so it gets a bit more acheivable since the factory already went and engineered it for you. Those cars are very rare but enough were made that all the parts are out there needed to do the swap. An original 4 speed equipped donor car would be ideal to provide all the info and parts etc... It's NOT a simple procedure... and trying to assemble one without a reference will be very difficult.
I own a 4 speed Monaco and an automatic car as well, and therefore know firsthand what the differences are.
Be prepared to drop the front subframe to weld on the Z bar mount. You will be required to cut the front floor out and weld in the proper 4 speed hump. There are many fiddly bits needed to do it right so again - research.