If you are a Yohji size 3 (size 48 or a really small 50), one of the best ways to get Yohji pieces is to stalk
wire.artist's sales threads over on Stylezeitgeist. He has the most amazing pieces in his collection and every now and then he does some spring cleaning. Some of the pieces have barely been worn and he tends to sell at great prices and can advise on fit and more.
His size 3 sales thread was put up a while ago, but it is possible a few of the items are still available for sale. It's also worth browsing just to see some of the pieces

You can find the sales thread
here. He also has this scarf blazer from AW08/09 -

that could also work for a size 4 (50)
here.
asobu also has some great pieces that he puts up for sale as well suitable for size 4ers on here or over on SZ as
chameleon.