My method was to use a freshly reformatted hard drive in an external USB enclosure, because I didn't have a large enough flash drive. It requires about 7.5GB (DVD-DL), so no, a 4GB won't work, but you can pick up an 8GB or use the external like I did. Follow the instructions at macinverse and it goes very quickly.
Core Storage versus resizing
… Will I need to decrypt my Lion HD in order to resize it? …
For 10.7 (Build 11A511): yes.
Whether a future release of OS X will allow greater flexibility, I don't know.
If you use diskutil to monitor backward conversion —
diskutil coreStorage list
— you may find that the amount converted eventually falls short, just a little, of the amount to be converted, with no further progress.
When you are certain that conversion is as far as possible complete, restart the system.
(AFAIK the logical volume from which the system was started will remain in coreStorage world until close down.)
This answer partly overlaps with an answer under:
Merging partitions, or using free space previously occupied by a deleted partition
After the disk no longer uses Core Storage …
… Old Snow Leopard install is in second half … to delete the Snow Leopard partition …
In this case you might use the mergePartitions verb of diskutil — "… All data on merged partitions other than the first will be lost. …". If you use diskutil in this way, proceed with caution (I once made a mistake that was destructive).
Hint
Whether you use Disk Utility or diskutil, habitually verify all volumes on a disk before and after manipulating its structure. In some cases an apparently routine operation may be followed by a need for minor repair, a need that is not immediately obvious.
Best Answer
I found out what happen. Its seem that the 4GB flash drive option isn't working. I've manage to find an 8GB flash drive and try again. This time no problem!.