Question: issues reading/writing to multibyte registers in AD4195-4 using the SPI interface. It can be read-modify-write single byte registers, but are unable to access multibyte registers within a single SPI transaction.
Answer: Regarding the SINGLE_INST (bit 7) in INTERFACE_CONFIG_B. The AD4195-4's SPI interface only supports exactly one register access per SPI transaction. It is always '1' as the device is only capable of single instruction frames for every low in CS pin.