Help: GC holder going to China

I'm a GC holder and will go back to visit China for up to 4 months. I was told by friends when I come back to the States, if the immigration officer asks me how long I have been away, I better lie and say "2 weeks". If I answer longer than that I might get into some kind of unnecessary trouble. Anyone who has experience on this please share your opinion here. I really appreciate that!