4M x 8 = L x W

BYTE-ADDRESSABLE:

4M = 4 x 2^20
   = 2^2 x 2^20
   = 2^22
   = 22 bits
   
WORD-ADDRESSABLE:

16-bit word size:
= 22 - log 2 (16/8)
= 22 - log 2 (2)
= 22 - 1
= 21 bits

32-bit word size:
= 22 - log 2 (32/8)
= 22 - log 2 (4)
= 22 - 2
= 20 bits

Formula: Byte-addressable bits - log 2 (word size/Width)