How many bits are required to address a 4g x 32-bit main memory if main memory is word-addressable? main memory is byte-addressable?

The answers are 32 and 34 bits

A byte is a memory unit for storage and a memory chip is full of such bytes. Memory units are addressable. That is the only way we can use memory.

A word is just a group of bytes (2, 4, 8 depending upon the data bus size of the CPU).