I agree, a 4 GB flash drive would not seem to be large enough to hold the image necessary. The Asus All-in-one I had contained a 16 GB install.wim file, which was divided, of course, to fit on the flash drive.
But it seems some OEMs are putting the install in more than one image file, so maybe it has something to do with that. This assumes we are referring to a Recovery Drive from Windows 8, and not an OEM Recovery drive, which may use the partitions already on the original drive.
Both of my Dell Recovery Drives, OEM and Windows 8, use about 8.3 GBs of the 32 GB flash drive.