How to add HiFi quality audio input to MacBook Air M4?

Ok, let's see if I can answer your question ...

... to start with here are some things to be aware of / watch out for:

• Your M4 MacBook Air supports USB-C/Thunderbolt ports. As long as the interface is class-compliant or has current Apple silicon drivers, it should work fine.
• USB-C cables can vary in quality (especially bus-powered models).
• Since you said "not mic input, no DSP, megabass/air etc., just flat audio input” you’ll want an interface with a proper line-level input. Often the input is labelled “Line/Instruments” or there’s a switch. Also ensure the converters are high quality (24 bit, 96 kHz or 192 kHz) to preserve fidelity.
• Some interfaces have extra features (Air mode, built-in DSP, console emulation) that you can disable — that’s good if you want flat. But the ideal interface will just give you clean conversion and minimal circuitry between source and computer.
• If you’re bringing in a line-level signal (from a mixer, outboard, etc.), ensure cables are balanced (if available) and avoid ground loops. Good shielding helps maintain fidelity.
• While your Mac will easily handle 24-bit/96kHz or even 192kHz, make sure your source is clean and you’re avoiding unnecessary up-conversions. Keeping the chain as direct as possible yields the cleanest result.

Ok, with all that said, here are some audio interfaces that you may want to consider:

• SSL 2+ MKII
• MOTU M2
• Focusrite Scarlett 2i2 4th Gen
• PreSonus Studio 24C
• Yamaha UR22MK3
• Arturia MiniFuse 2