Depends on your budget at well.
Probably the best bang for your buck is the 4R70W, used out of a '98-01 V8 Explorer AWD model. Quick adaptor and the Bronco transfer case bolts right on, or a junkyard Ford NP205 can be hung without an adaptor (it's just bigger, heavier, stronger, and getting hard to find, not to mention most will have at least one wrong yoke).
Modified or custom crossmember, modified/replaced driveshafts. A controller to run the transmission. And if your gearing won't support the overdrive, new gears. Speaking of which, what size tires and what gearing will you be running. And what engine as well?
In short there is no quick overdrive transmission that is a perfect exchange for the C4 you have now. It is a very doable conversion, but not a single part swap to do so. On the low end about $1k if you sourced parts just right, did the work yourself, and scored some deals and sold the old C4 for a good bit of coin. Ramp that up to a high dollar build transmission, put all new parts everywhere, pay someplace to do the work, Have a high end shop do the work, it could tickle the $10k mark. It's all over the map. This is more custom work than part replacement so you would need to look for a fab shop that does custom work more than an auto repair shop that just replaces stuff that went wrong. So you even need to pay attention to the kind of shop that is doing the work if you farm it out.
