Contact the nearest Japanese consulate and ask them about applying for a descendant visa. I would also work on your Japanese ability. That would help convince to give you the visa. But I think you should be able to get the visa.
The best info I can find is this,
http://www.ph.emb-japan.go.jp/visiti...nikkei-jin.pdf. It is from the Phillipines, but the process should be similar where ever you are. I think being a quarter makes you 2nd generation, but I have no idea. And honestly, the person stamping the visa probably has no idea either. Just get to work on getting your documents in order. Family trees, marriage certificates and birth certificates are going to take some time to put together. Good luck.