The only apps I’ve found that allow for any kind of mograph work in 4K on iOS devices are iMovie and LumaFusion (and LumaFX), and though LumaFusion is a startlingly powerful editor, it’s not quite up to full snuff for sophisticated mograph. You can do a lot in this app, and I’ve done a lot where I prep elements in something like Affinity Photo, Procreate, or Autodesk Graphic at a resolution to work in 4K, but if you’re looking to do something super intricate, you may need to bounce out to your computer and AE or Motion. At the very least, expect to do some pre-renders of elements inside LumaFusion to get more layers in your L3s.
If you’re finishing in 1080P, there are several apps that can produce some truly amazing motion graphics on iOS. First among them is Bricolage; it’s a node-based graphic editor, and it’s really powerful. Produced by the same developer is MotionGraphix, which is very similar but has some different functionality. Core Animator is promising, but not fully realized. (Core Animator is more of a developer tool than a media production tool, but it’s still quite useful.) Animation Pro can be pressed into mograph as well, and it allows for all sorts of animations that aren’t possible in any other app I’ve used. Alas, though, these are all limited to 1080P. I suspect more apps will eventually embrace 4K, but for now, it’s a frustrating, limiting world....