Banana PI BPI-R1. It's $60 + shipping, but has 4 LAN ports + 1 WAN port, all GB. There's a newer R2 model, but I can't find it for sale anywhere.

http://www.banana-pi.org/r1.html

https://www.aliexpress.com/store/product/Newest-arrive-BPI-R...

If you can get by without a powerful CPU, lots of memory, etc, there's lots of cheap multi-NIC routers that will run openwrt/LEDE.

I think the TI chip used in the beaglebone has support for multiple /ports/ via an RGMII switch or so.

It's been a while since I messed with that though.