Well, apparently, the people arrested are not believed to be those who murdered him. U.S. authorities believe Berg was kidnapped by one group and then sold to a different group. The group that bought Berg is the group believed to have murdered Berg. These four could be charged with kidnapping and various criminal conspiracies, but I don't see how they could be charged with murder--at least if U.S. authorities are correct in their belief that those arrested sold Berg to another group.
The crime was committed on Iraqi soil, so under traditional international law, the matter should be left to Iraqi authorities. However, since the U.S. IS the Iraqi authority right now, it's a bit unclear what this means.