Yes, I know these things are a little confusing when first starting out!
Looking at the App Store just now, I see that 4096px canvasses are only possible when using the older ArtRage app on an iPad Pro. On other devices the max size was/is 2048px.
The new ArtRage Vitae Mobile will allow the larger 4096px canvas size on any iPad, along with some other enhancements and updates over the original app.
If you have only recently purchased ArtRage from the app store it might still be possible for you get a refund (I don’t know what exactly the window of opportunity for claiming a refund is in the App Store, sorry) and then buy the new ArtRage Vitae Mobile version if you wanted.