This could be tricky. Pulling the year out of the date is the easy part.
Here's the problem: The standard Vorbis Comment field name for dates is DATE. Mp3tag maps the internal name YEAR to the standard DATE field. As long as that mapping exists, you can't write both DATE and YEAR.
You can remove the mapping (never really tried it and I don't feel like testing it) in Tools > Options > Tags > Mapping. This should allow you to work with both fields, but I guarantee that it's going to get very confusing. What you'll end up doing is writing the year to the YEAR field. But since that's not a standard field name, it's not likely to be recognized by any software that you use, and you're still going to have YYYY MM DD formatted data in the DATE field.
What I would probably do if I wanted to preserve the YYYY MM DD data is first copy it to a non-standard field, maybe ORIGINALDATE or something similar. Then convert the date within YEAR to only keep the year portion. This would require an action group containing two actions. The following should work:
Action type: Format value
Format string: $iflonger(%year%,4,%year%,%originaldate%)
Action type: Guess values
Source format: %year%
Guessing pattern: %year% %dummy%
The first action will copy any dates longer than four characters to the ORIGINALDATE field. It won't create the field if the date isn't more than four characters, so it should also avoid overwriting it if you run the action group more than once.
The second action leaves only the first part of the date (anything before the first space). Running it more than once will be safe and will leave the DATE field unchanged on subsequent runs. If your dates are sometimes formatted other than YYYY MM DD (say YYYY-MM-DD, or M-D-YY) then it will require something smarter, probably using a regular expression.