@Anonymous User
Bit of cross-discussion about NOW TV usernames and Channel4 IDs, but yes you are right your Channel4 ID consists of an email address with a password, and you do need to enter a full email address when logging into the All4 app (otherwise it reports invalid).
Obvious question first, it's definitely a Channel4 ID you're using to log into the All4 app, not your NOW TV login? And have you checked it works by logging into the Channel4 website with it?
I've just signed out and back into mine, it doesn't have an issue. As suggested if you haven't already try deleting the app by highlighting it on your My Apps screen but instead of pressing OK to open the app press * and choose remove app. Then remove the power from the box for a few seconds, reconnect back up and add the app again from the App Store.
If you still have trouble logging in after that I'd try contacting Channel4 direct to see what they suggest.
https://www.channel4.com/4viewers/contact-us
