Game doesn't run with more than 4GB ram allocated

When I tried to allocate 6GB of my RAM to minecraft through the JVM argument "-Xmx6G -Xms6G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -client -d64" The game instantly crashes on launch. Game Info window doesn't show anything and I only get the message "Crash: An unexpected issue occurred and the game has crashed. We're sorry for the inconvenience."
Mind you that if I change the JVM to "-Xmx4G -Xms4G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -client -d64" the game runs fine.