Please ignore banana-pi's Wiki when using OpenWrt from openwrt.org, the SinoVoip firmware is based on OpenWrt somehow, but it's very different. It's a "Raspbian vs. Debian" situation, just that (unfortunately) SinoVoip called their fork "OpenWrt" which is the source of the confusion...
Please refer to https://openwrt.org/inbox/toh/sinovoip/bananapi_bpi-r4 instead. Installation to SPI-NAND and eMMC works like on the BananaPi BPi-R3.