An A4000 is *much* easier to expand than an A1200.
There's lots of ethernet cards out there that just plug and go, such as my Ariadne, the Hydranet, and lots of others.
Then you can have a Deneb (again, like I do) and connect a cheap USB ethernet card.
Also, you can have a Prometheus (which, once again, I have) and use a bog standard RTL8029.device which you can get for a fiver. You can probably get wireless for that too but I've not looked.
I have an A4000, a B2000, an A1200 and an A600 all ethernettable, and have had no difficulties with any of them.
It's no more difficult to get an A4000 online than an A1200, in fact I'd say it's easier because you don't have to sacrifice your PCMCIA slot.
Edit: For reference, I'm pretty sure there's no SANA-II drivers for ethernet on the SCSI device, I'm afraid. However, I believe you could use a Mac emulator to go online with it!