For SPI Flash use in x4 mode Quad Enable bit must be set in nonvolatile Status Register. The commands for this depend the SPI Flash Vendor and/or Flash type.
Checking if the Quad Enable bit is set is possible from u-boot
> sspi 0 16 35
if result is 0002 then the nonvolatile Quad Enable bit is set
It is also possible to write the Quad Enable bit, 2 commands are needed
> sspi 0 8 06
> sspi 0 24 010002
The above will set Quad Enable bit in non volatile status register