-They may have a lot trouble at the port of entry if they just use their green card to enter US. They will have big pressure to deal with the IO questions. Even if the IO allow them to enter US, they will face the immigration judge. So, I don't suggest them to do that.
If they rally had illness which prevented them from return to US for the entire last four years, they can apply for the SB-1 visa in the US Consulate in Guangzhou to return to US to reinstate their green card. But they need to provide strong evidences to justify the SB-1 visa application.
Otherwise, they may just abandon their green card in any one of the US Consulates and then, apply for B-2 visa to enter US. If they want to get the green card again, you can sponsor them again as you did first time.