I'm just curious

[djordje.janjic29]

So, I start google it for some things and suddenly I see that someone is selling new sensor for 5D3 and then I have idea. 5D3 and 5DIV use same size sensor, so can you put a sensor from 5DIV to 5D3? Has anyone ever managed to do something like that, like upgrade for your PC, I'm just curious? 

[Walter Schulz]

Sorry, that's absolutely not how it works.
There was a cam designed for this: https://en.wikipedia.org/wiki/Ricoh_GXR

[josepvm]

5D3 sensor is a pure analog device, that feeds analog image data to an external chip that converts it to digital data.

But 5D MkIV sensor has on-chip A/D converters, so the output is digital data.

Conclusion: you can't use a 5D MkIV sensor in a 5D MkIII camera.