Which recovery method did you use? Which version of Disk Drill did you run? Were these files protected by Recovery Vault or Guaranteed Recovery?

Also, have a look here: http://help.cleverfiles.com/can-i-recover-a-specific-file-with-disk-drill/

And finally, AI (Adobe Illustrator) files ARE supported by Deep Scan, if not all files were recovered, this doesn't mean the format is not supported.

these files were not protected and my ver. is 1.5.167

I think I did quick scan. :-(

What I'm wondering is how to recover those 4 files again.
I mean, since I already recover the files, Disk Drill can not find originally deleted files.
and the recovered files are just shells. No data.

What do I suppose to do now? no way to recover?

Have you checked the article I provided the link for? It explains how data recovery works. There's no guarantee that everything you deleted can be recovered. Trying Deep Scan is also important, Quick Scan will only locate those files that are still listed in file system. Deep Scan may get more deleted items from your disk, if they are recoverable.

I have only recently purchased disk drill pro to get back my files. I did a deep scan of the computer to recover all deleted files/ folder from a total erase.

I have found that all ai and idnn files are corrupt. Is the partition scan and quick scan going to be any better?

You should always start with a Quick Scan, and only afterwards proceed to Deep Scan. While there's no guarantee that everything can be recovered, Quick Scan has less chances of producing corrupted files, than Deep Scan, but may not find files that were gone long ago. More details in our knowledgebase: http://help.cleverfiles.com/recovered-items-corrupted/