Sophos 105w Rev 3 (or 115w Rev 3, or 106w) ticks almost all the boxes.
- four ports, one of which is "twinned" (can be accessed via RJ-45 or SFP)
- single-radio AC-standard Wi-Fi card (Qualcomm Atheros QCA986x/988x)
- desktop size (footprint approximately 9 x 6 in / 225 x 150 mm)
- built on Intel Atom E3930 processor with 2 GB or RAM and a 64 GB SSD, so runs OpenWrt or any mainline Linux (RAM and SSD are removable and thus upgradable)
- don't know if Coreboot is a possibility
The photo below shows an SG 105w; there's also an XG 105w that's hardware-identical, but looks different (just a tad larger, and the case has rounded edges).
I have a couple of these lying around with OpenWrt already installed, so give me a shout if you want one...