What to do if you get 'Permgen space' crashes

[apn999]

java -Xmx4G -Xms1G -XX: PermSize=256M -jar FTBServer-1.6.4-965.jar nogui
That is the Batch File command i use to launch the server. When i launch after about a minute the command prompt stops at "[ForgeModLoader] Unloading Dimension 1." Not too sure why.

 

[Boss 44]

Thanks, you helped me so much!