Morning Neil,
2 bodies
IMHO, logic would dictate that if you have two bodies, one as a back up to the other, that they be both the same. You will then have a seamless transition from one to the other not only in terms of the UI and controls etc, but also in the “feel” of the camera in your hands.
The most cost effective solution, whilst retaining top quality, would be 2 x D800 bodies, (E or normal), OR, as you have a D4S, two D4S bodies.
As you have the budget, and as you have a D4S, surely another D4S is the way to go and I would think that even keeping the D3S alongside the D4S is better than throwing a D800E into the mix.
Alternatives are:
1). Keep what you have which would seem to many as a really good option.
2). Buy another D4S and sell the D3S … same "net cost" as throwing a D800E into the mix.
3). Sell what you have and buy 2 x D880E’s and two grips.
I know that you are keen on “bird” images; the DX D7100 will give you 50% extra reach with more pixels on the image than the FX sensor in the D800 used in DX mode will give and you also have the option of a further in camera x 1.3 crop if you want to keep files sizes lower. DX does not necessarily mean a lower quality or a cheaper body.
If you want to throw another body into the mix that you have consider the D7100 as it may be a better and certainly a cheaper option than the D800 and it is a camera that you could use specifically for birding, albeit with a (more) different UI versus the D4S than the D800E has.
A lot of us would welcome having your “problem” - tough times
Good luck