WAS 8.0.0.4 does not start after entering generic JVM arguments (CLM 4.0)

With only 8 gigs of memory I would get rid of the -Xgc:preferredHeapBase=0x100000000setting
That tells the JVM to use  memory above the first 4 gigs, so that may be cutting it kind of close.  Especially since it requires the memory it uses to be contiguous.
You could also try to use a smaller xmx and xms setting

* I originally wrote to remove the  -Xcompressedrefs setting 
which is incorrect it should be the
-Xgc:preferredHeapBase=0x100000000 setting as corrected above

Usually when WAS fails to start properly after setting a property, it means the property was not set properly, or set to a value too large for the server to handle.  The setting can usually be adjusted by modifying the server.xml file rather than deleting the profile and starting over.  I suggest checking the systemout or systemerr logs for errors if this happens again so you can determine was is causing the problem.

Typically, this can happen if you set the JVM max heap size to a value larger than 1/2 the physical memory available on the system.  For example, if your server only has 4GB of RAM and you've set the max JVM heap to 4GB, WAS will not start under that condition since you need native memory available for the system to function properly.  It is also not difficult to make mistakes when setting the generic JVM arguments (ie - entering a ; instead of a :, omitting spaces between the -X properties).  I can't say exactly what caused the problem based on the information provided, but I hope it helps,

-Xmx4g -Xms4g -Xmn512m
-Xgcpolicy:gencon -Xcompressedrefs
-Xgc:preferredHeapBase=0x100000000

Let me know if the abovs is fine and the max JVM heap is set to 4096