open 'data.ext4.tar' file

[dlw]

How is 'data.ext4.tar' file opened in 12.10?
Archive Manager will not open it.

dlw

[Impavidus]

Archive manager (which is, at least on my machine, actually called file-roller, but can be opened from the file manager by rightclicking on a file and selecting "open with archive manager") should be able to open .tar files.

First make sure it really is a .tar archive. The file manager (or at least thunar, I'm not sure about nautilus) assumes it's a tar archive whenever a file is called .tar, but this is no guarantee. Type in a terminal
file /path/to/data.ext4.tar (substitude the correct path)
It should give us the exact file type. If it's not something like "tar archive" this file isn't what it pretends to be.

If it is a tar archive you can unpack it in a terminal with
tar -xvf data.ext4.tarIt's best to first put it in a directory of its own. If not well-behaved, tar archives can produce a lot of files in the current directory.