Checkstyle: count tabs as four characters, not eight

config/checkstyle/checkstyle.xml

@@ -2,6 +2,7 @@
           "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
           "https://checkstyle.org/dtds/configuration_1_3.dtd">
 <module name="Checker">
+  <property name="tabWidth" value="4"/>
   <property name="charset" value="UTF-8"/>
   <module name="LineLength">
     <property name="fileExtensions" value="java"/>