Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:2/4
K:G
['GGAA B2 G z']  
X:1
L:1/8
M:2/4
K:G
 GGAA | B2 G z | GGAA | B2 G z | BBAG | B2 B2 | AAAB | G2 G z |]  