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

Method #1

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

Method #2

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