ANTLR grammar not loading

I'm working on an ANTLR4 project in Java and I have some troubles with the cmd. I've created both the grun.bat and the antlr4.bat files and the classpath is correct. I have this grammar called "Grammar.g4". Whenever I type in cmd "antlr4 Grammar.g4" it generates my other files (lexer, parser, etc.). The next step is to prepare those files in order to get the test going with "javac Grammar*.java". My problem is that when I type "grun Grammar foo_start -gui" I get this error:

 Warning: TestRig moved to org.antlr.v4.gui.testRig; calling automatically
       Problems calling org.antlr.v4.gui.TestRig.main(args)  

Thank you for your time!