Thanks for the help here, davidking.
Rustic, I checked this on the All 4 website, and they've mentioned that you can watch all the latest catch up, free box sets, plus a huge library of recent, classic, and online shows including Shorts, but it is not currently possible to cast the Watch Live service nor Clips and Extras. Channel 4 must acquire specific rights for some programmes in order to show these on a particular device, therefore some shows may not be available to cast on Google Chromecast. You can see this information here.
I hope that clarifies your concern and feel free to let us know if you have other questions and concerns.