Like Rami, I think the fourth commandment is packed with meaning, some obvious, some less so. I find the commandment speaks to the following matters: time, rhythm, permission, community and idolatry.
Time--the commandment sanctifies time, setting aside one day as a way of driving home that all time belongs to God.
Rhythm--the created live best when they set the rhythm of their lives by the divine rhythm.
Permission--God grants us permission to rest, setting himself in opposition to many cultures (including our own)which tend to devalue rest.
Community--as Rami suggests, the commandment cast ripples, thus challenging the ancient and modern tendency to dehumanize others.
Idolatry--offering the Sabbath to God calls us back from various idolatries (finding identity only in our work, treating work as the ultimate source of security, etc).
The Fourth Commandment is subversive, in the best sense of that much used term. Taken seriously, it undermines many an established viewpoint and practice. Just as importantly, it offers an alternative way of ordering one's life.