well at ~16 megabytes/sec for 12X thats expecting 64MB/SEC for all 4, That can be done(most drives reach that speed, even older ones), but you have to take into consideration that latency of the drive having to jump from place to place(since its reading 4 different ISOs or sets of files) to feed all burners. I am sure you are just causing all burners to wait between seeks for others.
2(since 1 drive handles 2, i guess 2 should handle 4) or more hard drives may help this.
All in all, just go 2 at once.
EDIT, did not see that hunter315 already covered this. my bad