Ubuntu One is a sync service, not a bckup service. When you add files from 4 coumputers to Ubuntu One:
- Do you use the same user-id (lets call the user-id "LRBellmore") and password to configure the Ubuntu One app in all the computers? I am talking about Ubuntu One user-id.
- Do you copy the files in the "Ubuntu One" folder in your home folder,
or upload the files using the web interface?
If the answer to Question 1 is yes, it may take sometime, but files from computer A will show up in computer B, C, and D if:
1) the same user remains logged in, on the other computers (B, C, and D)
2) Internet is working, and
3) Ubuntu One is running in them.
As for what happens if a computer fails, depends on how the computer fails.
If the user "LRBellmore" is logged into the computer A and Ubuntu One app is running (as it should) and some malware (or a toddler) deletes files from a synced folder like "Ubuntu One" folder. The app will send this information to the server and the corresponding file will be deleted from the server. When "LRBellmore" logs on to computer B, and connects to the Internet, the file will be deleted from computer B.
If the computer A has a second user (Lets call him "EvilTwin"), and EvilTwin logs into EvilTwin's account and somehow manages to delete files from a synced folder like "Ubuntu One" folder of "LRBellmore," then the files will not be deleted from the Ubuntu One server at that time, as "LRBellmore" was not logged in at that time and the Ubuntu One was not running with "LRBellmore" credential. However, when "LRBellmore" logs into the computer and connects to the Internet, The Ubuntu One app will notice that files are gone and will sync those changes to the server, and eventually to the other computers.
If the computer was turned off and falls in a swimming pool, then the user "LRBellmore" was not logged in and the Ubuntu One app was not running. So no information was sent to the server. So no file in the server will be deleted. When the user "LRBellmore" logs on to the computer B, the file will continue to exist there as well.
If the computer was booted from a Live CD and reformatted, the user "LRBellmore" was not logged in and the Ubuntu One app was not running. So the file will remain in the server and in computer B, C and D.
The same principle holds for dual-boot Windows and Ubuntu, on the same computer. Once the files are synced to the server from the Windows boot, you have to boot Ubuntu and log-in to your account so that the files can sync between the server and the Ubuntu's local "Ubuntu One" folder. If you delete the Windows partition while booted from LiveCD or Ubuntu in the other partition of your hard drive, Ubuntu One will not be able to sync with the Windows part of the computer and no files will be deleted.
Hope this helps