Don't force IPv4 network stack if IPv6 is specifically requested (#4547)

src/main/java/net/minecraftforge/fml/common/launcher/FMLTweaker.java

@@ -46,7 +46,10 @@ public class FMLTweaker implements ITweaker {
 
     public FMLTweaker()
     {
-        System.setProperty("java.net.preferIPv4Stack", "true"); //Lets do this as early as possible. Vanilla does it in Main.main
+        if (System.getProperty("java.net.preferIPv4Stack") == null)
+        {
+            System.setProperty("java.net.preferIPv4Stack", "true");
+        }
         try
         {
             System.setSecurityManager(new FMLSecurityManager());