What does apostrophe mean for a register description in a datasheet e.g. 4'd5?
4'd5 indicates a four-bit value, expressed in decimal as 5. It's the same as 4'b101 (1012 = 510).
Likewise, 4'hF is a four-bit value indicating 0xF in hex; octal may also be seen (e.g. 4'hF can also be denoted as 4'o17)