Hi, that should be easily doable.
You’ll need a compatible audio interface though.
I use the Presonus 1818VSL, which would do what you want, it has 8 audio outputs which could be used as 4 stereo outputs.
You’ll also need some sort of USB hub between the iPad and the audio interface, to overcome a USB power limitation that all iPads have, which is bypassed when using a USB hub.
To get the stems into Cubasis you can use dropbox, iTunes or a file manager app on your PC/MAC like iFunbox which gives you access to the file system on your iOS device.