What iPad is best to use for graphic design?

Air 4 or iPad Pro

You can directly compare iPad models here:

iPad - Compare Models - Apple (UK)

What’s best for you is subjective - and only you can determine the specifications will meet your needs.

Perhaps consider that the iPad Pro models represent the current state-of-the-art “powerhouse” of iPad performance- whereas the iPad8 is the budget-conscious entry-level model. The new fourth generation iPad Air sits between these two extremes.

You might be best advised to an iPad with as much internal storage as you can afford - as the internal storage is fixed and cannot be later updated or expanded. A good rule of thumb is to consider what you think you’ll need, add a healthy margin - and double it.