Hello,
First you have to get the group2 node list then try to get the group4 node list from group2 node.
Here is the code,
XPathNodeIterator group2Nodes = this.CreateNavigator().Select("/my:myFields/my:group1/my:group2", this.NamespaceManager);
foreach (XPathNavigator group2Node in group2Nodes)
{
XPathNodeIterator group4Nodes = group2Node.Select("my:group3/my:group4", this.NamespaceManager);
foreach (XPathNavigator group4Node in group4Nodes)
{
//your code to get group4Node values.
}
}