Don’t use Render and Replace for this. Not its purpose.
Select the clips you want to create proxies for in the project panel. Right Click and Select Proxy/Create Proxies. Important: Select a preset that matches the aspect ratio of the original footage. Media Encoder opens and creates your proxies. They become attached when the encode finishes/
Make sure the button “Toggle Proxies” is included under your is included under your Program Monitor. If not, add it by clicking the plus sign and add it from the window that appears.
Toggle the Proxies on and you are playing the proxies instead of the original 4k files.