This should be possible if you use both I2S ports.
The first I2S port has 5 data pins, so it can do up to 10 channels. 3 of those data pins can be either input or output.
The second I2S port has 2 data pins, so it's always just stereo input...
No, as design and spec tends to change during the prototype stage, I don't publish until I have something worth releasing.
The PCBs won't really be suitable for DIY construction due to the QFN CODECs. I'm exploring PCBA with JLCPCB and may...
Quick followup, O_C T4.1 is now in beta testing. Here's a photo of one of the beta test units being calibrated.
Full open source release is likely soon, of course depending on feedback from beta testers in the coming days. But so far...