How does MIPS ( or any other multi-byte word arch ) fetch a word?
MIPS can load a 32-bit (4-byte) word in a single instruction (load word, LW). The fact that each byte of the word is individually addressable doesn't affect this.  Effectively, the instruction says "Read the four bytes beginning at this address", not "Read the byte at this address.  Increment the address by one.  Read the byte at that address.  Increment the address by one.  Read the byte at that address.  Increment the address by one.  Read the byte at that address."

By way of an analogy, you can photocopy any two facing pages of a book simultaneously, even though each has its own number.