Hi,
yes the vmdk and the flat.vmdk should be enough to re-create an Entire VM. The Most Import file is the flat.vmdk, which will not be visible on GUI but if you SSH into the Host and Cd to the data store you should be able to see the
flat.vmdk. Even if you lose all the files and have Just the flat.vmdk you can re-create the entire VM with data.
Ideally when you copy or move the .vmdk (descripter ) it also moves or copies the flat.vmdk on the back end.
And yes you can import the version 4 to your 5.5 without problems.
Thanks,
Avinash