Worked in China 5 years, returned to USA last July 1st, working visa is still valid until April 5th

I worked in CHina for 5 years in Beijing and when my contract expired I decided to return home to be with family. I have a possible opportunity to do some business in Beijing next month but my working Z expires on April 5th, and my question is if I return for business in March is it still okay? It reads valis until April 5, 2014. You can reach me at my email address: karljweaver@gmail.com, my mobile 1-425-260-4378. I am in the Seattle area, and so I just wish to know if I go back now, even though I am not working again in a new company if it is still valid until April 5th? Thanks for your feedback.我在北京高科技公司上班5年，我的护照有中国工作签证，但是在4月5日会到期， 能不能在4曰5日之前到中国做事不用重新申请新的签证吗？
Regards,
Karl J Weaver 魏卡尔