failing to print not matched lines from second file

[tester_V]

with open('f1.txt', 'r') as file:    
lines = set([line.strip().split(',')[3] for line in file]) 
with open('f2.txt', 'r') as file:    
for line in file:        
line = line.strip()        
if line.split(',')[3] not in lines:            
print(f'{line} NO MATCH')

Thank you! It works I just confused about the square brackets in this line:
set([line.strip().split(',')[3] for line in file]).
Thank you again!