Hi Andy,
Hmmm, where to start. It's a good question. One place to take a look see is
Jobs in Japan
This is where a lot of jobs get advertised - select the industry it/design/media and see what comes up. A lot of this is about contacts and unfortunately I'm not in your industry. Some of the magazines here could be good starting query points - there's one called Metropolis and called Weekender.
There's also a group called
DesignFesta - they are definitely worth checking out I reckon.
As to the 4 year degree - it's required if you are going into something like English teaching but if you have experience plus a specific skill that is sought after then I know people who have been fine with the 3 year qualification.
Sorry I can't be more help - but I would definitely contact DesignFesta and see if they have any suggestions.
All the best - Honor