We are no longer required to keep the law, for Christ kept it for us.
But, the Holy Spirit inside us should still create in us a desire to keep the MORAL law, which has never been destroyed. The 4th Commandment falls under the moral law and simply has a slight change under the NT in that with Christ's Resurrection, it is now Sunday we meet to celebrate his resurection weekly, rather than Saturday. But, Sabbath Keeping is still required.
I suggest you read this article defending Sabbath Keeping:
http://www.reformed.com/pub/sabbath2.htm
I agree with this...mostly. But where in scripture does it say the sabbath was to be changed from the 7th day to the 1st day?