A: Losing weight?
B: I've lost a little.
※ I wonder which is correct.
1. Losing weight? = Are you losing weight?
2. Losing weight? = Have you been losing weight?
You have a very good question! That sentence is confusing, even to me. As a native speaker, I would never simply say "Losing weight?" However, based on B's response, I think that Losing weight? = Have you been losing weight? Otherwise, the tenses between the two speakers wouldn't agree.
※ I would like know the difference between "3" and "4".
3. Have you lost weight?
4. Have you been losing weight?
The difference between these two are very subtle, but there is a slight difference to the discerning eye. When someone asks: "Have you lost weight?" they are placing emphasis on the total amount of weight that you have lost as a whole. Someone might respond: "Yes, I lost 10 kilos this year!"
When someone asks: "Have you been losing weight?" they are placing the emphasis on the process and duration of the weight loss. A typical response might be: "Yeah, when I was in the hospital, I had been losing weight every week until I was able to eat solid foods again."
In summary, #3 emphasizes the total amount of weight whereas #4 emphasizes the process/duration of the weight loss.
Hope that helps!