This is said to contain the same DSP as the desktop version of AmpliTube 3, which should theoretically result in the same quality of sound. There's also the new Cab Room (a $9.99 in-app purchase), which enables you mic-up cabinets in 3D using two microphones, and 16 new mic models are available for purchase ($9.99 for the lot or $0.99 each).
On a more creative tip, there's now a 4-slot Looper ($9.99 as in-app purchase), which could be useful for getting ideas together quickly, and you can now use up to six stompboxes in your rig. These can be positioned via drag 'n' drop.
AmpliTube 4 is available now from the Apple App Store priced at £14.99/$19.99 (existing users will be upgraded for free), while you can buy all the new features in a bundle for $19.99.