It is a slangy dialog - probably less than one percent of Koreans can understand it (I myself googled it). 짬's original meaning is free time, like a short lull or break taken out of a tight schedule. We say 짬이 나다 (find some time to do something else) or 짬을 내다 (make time for something). In your example, it is using 짬 in a different sense coming from 짬밥, a slang term indicating how long one has worked in some capacity (i.e. 경력 or 관록), especially in the military service. In this meaning, 짬이 좀 차다 (차다 = be filled) seems to be widely used for "to gain fair amount of experience". So the dialog may be translated as: A: 이제 짬 좀 찼고 => Now I've gained a fair bit of experience. B: 무려 4개월 짬 => (Right) No less than four months of experience. Most likely B is being sarcastic about A calling himself experienced only after four months, like "Wow. Four months. Am I impressed".