The SoC doesn't support much more than 2G, the Allwinner Wiki says 3G is the maximum but you probably want to just use two memory chips to keep costs down.

Is there an ARM motherboard that accepts more that 4GB?

You could get a Xilinx dev board and just ignore the FPGA features, this [1] one will accept at least 6GB.

[1] https://www.xilinx.com/products/boards-and-kits/zcu104.html