Sideload Apps on Chromecast (4th Gen) with Google TV

Method #1

Go to Settings > System > About
Click on Android TV OS Build until Developer mode is activated
Go to Settings > Apps > Security & Restrictions
Enable Unknown Sources
Install Send Files to TV from Play Store
Apps > Start Send Files to TV and follow instructions
Download the apk you want to your smartphone
Start Send Files to TV on your smartphone and follow instructions
Go to Receive in the Send to Files app on your Android TV and double press your transferred apk to install it

Method #2

Go to Settings > System > About
Click on Android TV OS Build until Developer mode is activated
Go to Settings > Apps > Security & Restrictions
Enable Unknown Sources
Install Solid Explorer from Play Store
Apps > Start Solid Explorer
On the top left click on the three lines
Navigate to bottom until you find FTP Server and open it
Click Start on the Right
On your PC, open File Explorer
Write the Server Address in the Address Bar and press Enter
Copy the apk files you want to install to Download folder
Go back to your Solid Explorer Window on your Chromecast with Google TV
Press the left top arrow then navigate to Download folder
Now click on the apk you want to install and choose package Installer and install