Answer from cs61c-ce (aretha samuel 15797004) for Question 2
the 26 bit field in jump instructions is also a word address, meaning that it represents a 28 bit byte address.  Since the PC is 32 bits, 4 bits must come from someplace else.  The MIPS jump instruction replaces only teh lower 28 bits of the PC, leaving the upper 4 bits unchanged.