XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml("xml>" + s + "/xml>");
xmlDoc.SelectSingleNode(@"http://b4").InnerText = "修改后的b4";
Response.Write(Server.HtmlEncode(xmlDoc.DocumentElement.InnerXml));

string s = @"a1>a1的值/a1>a2>a2的值/a2>a3>a3的值/a3>a4>b4 id='b4'>b4的值/b4>/a4>";
HtmlDocument hxmlDoc = new HtmlDocument();
hxmlDoc.LoadHtml(s);
hxmlDoc.DocumentNode.SelectSingleNode(@"http://b4").InnerHtml = "修改后的b4";
Response.Write(Server.HtmlEncode(hxmlDoc.DocumentNode.InnerHtml));