WASHINGTON (Reuters) - The U.S. military said on Wednesday it has told families of four American soldiers killed during an ambush in Niger on Oct. 4 that it expected to complete its investigation into their deaths in January.   Families were informed that (the Africa Command s) investigation team will travel to locations in the U.S., Africa and Europe to gather information related to the investigation,  the Pentagon said in a statement. It said the probe was to be led Army Major General Roger Cloutier, the command s chief of staff, and families would be told if more time was needed. 