The Panasonic GH4 bundled with the YAGH Interface Unit kit is, as of the time of writing, in-stock and ready to ship for its starting price of $3300 by Kenmore Camera on eBay. As of the time of writing, there are 3 units available. This bundle is a savings of $400 versus being the GH4 and the YAGH separately (camera is $1700, YAGH is $2000 on their own)...
UPDATE: this bundle is now in-stock at B&H Photo as well for its starting price of $3300!
Speaking of which, the YAGH is now in-stock on its own for $2000 at B&H Photo with free expedited shipping. It is also for the same price with free 2-day shipping at Adorama.
You can do DIY checks at any time using the Panasonic GH4 stock status embedded on the main blog.