To my surprise, the Kirk BL-D3 bracket that I kept when I sold my D3X, fits my new D4 almost perfectly - and the mismatch doesn't matter in the slightest!
OK, so I was about to order a D4 bracket from RRS, as they seem to be the only source of shipping product right now, but had my D3 (with one of the Kirk brackets installed) on the bench alongside the D4. Mmmm! I wondered just how different they are so I went to the cupboard and retrieved the "old" bracket.
The ONLY function which is inaccessible is the "Peripheral socket" to which accessories, such as the WT-5, connect. I don't see how it would be possible to use such an accessory with the camera in portrait orientation while mounted using an L bracket anyway!
All the rubber covers to connectors other than the peripheral socket, can be easily opened and closed with the bracket installed.
The bottom line is that there seems to be absolutely no need whatsoever for a different bracket.
I'll bet that a LOT of us are upgrading to a D4 from D3 series bodies AND that many already have L brackets for them. Although I haven't had my hands on such a bracket from RRS, I can't see how it could be significantly different to the Kirk.