PSTRB signal of APB indicates the valid write byte lanes.
If the width of PSTRB is 4,
Must I use only 4'b1111 or 4'b0000 of PSTRB before APB4?
Can I use 4'b0110 of PSTRB after APB3?
If you are trying to connect an APB4 target with a PSTRB input to an APB3 (or earlier) source which doesn't have a PSTRB output, you would have to drive PSTRB=4'b1111 for writes and 4'b0000 for reads (spec says PSTRB must not be active for reads).... so basically use PWRITE to drive the target's PSTRB input bits.
For an APB4 source it can drive PSTRB to any value, including 4'b0110.