Its actually not a bad way of handling it, as while OSX may be more than 4GB, only a tiny fraction of that will be read at boot or general usage. The standard install includes all of the additional cruft and applications that are a part of OSX and not used frequently.
The hard disk has no knowledge of where files are, or even what filesystem its running, it simply copies the most frequently accessed sectors of the disk into the flash memory, so it may in fact copy part of a file if not the whole file.
Also means that if in a RAID 0 array, it should still cache the most used sectors correctly across both disks and in effect act as an 8GB flash cache (or more given more disks).
That's what I'm aiming for for my laptop once I get the second 500GB disk and Optibay.