var input = "My phone is 1234567890 and my office is 7894561230";

var output = "My phone is <u>1234567890</u> and my office is <u>7894561230</u>";

String input = "My phone is 1234567890 and my office is 7894561230";
String regex = "\d{4,}";
String output = input.replaceAll(regex, "<u>$0</u>");
System.out.println(output);