you want to use the Apple Camera adaptor .

for iPads with Lightning connector:

for older iPads:

This gives you a direct and stable USB MIDI connection.
I dis-recommend 3rd party adaptors. The Apple adaptor is worth the money.

you can also use it to connect class-compliant USB audio interfaces, such as this cheap one:


but iOS supports multi-channel interfaces too.

If you connect a powered USB hub to the iPad, you can e.g. connect an audio interface and the A4, and a keyboard…