About 2 months ago I asked myself the very same question. Save a little more and get a 1DX or buy a 1D4. I finally bought the 1D4 as an upgrade to my 7D. The main reasons were that it was a hell lot cheaper (still it was a load of money), because of the f/8 autofocus, 1.3 crop, very good autofocus and I decided for my self I did not really need the newest technology. I'm not a pro, and I'm not a millionaire. What also helped me decide, was I got the chance to shoot half a day with a 1D mark III. This was such a great experience that I just couldn't wait any longer to upgrade my 7D.
Like you, I use the 1D4 for wildlife and birds. With a 500 f/4, the camera performs really really well. With the 1.4x extender it performs good too, although the AF is a bit slower. Using a 2.0x extender (and f/8 autofocus) is only really usefull for subjects that are moving very slowly, or not moving it all. The AF is pretty slow and hunts a lot. IQ takes a hit too. For close-up portraits of birds or other wildlife it is a really great option to have though!
For me the 1D4 was a great choice. If you're not shooting many small or shy subjects, you'll probably don't need the 1.3 crop and f/8 autofocus. A tough choice. Buying a 1D4 and a 5D mk. III would be a great option too, you would have the extra reach of the 1D4 and the fullframe sensor of the 5D3. Plus 2 camera's, so you can carry 2 different focal lengths with you.
By the way, what lens(es) do you use?
If I had more money, I'd probably not sell the 1D4 but use it together with a 1DX. Use the 1D4 when I need the reach the 1DX cannot deliver, and use the 1DX for the rest
Check this topic
for a couple of beautiful examples Gary made with the 1DX. If you're able to get close to the animals or have a long 600mm lens, the extra reach of the 1D4 is not always needed. Although, I am not able to get these kind of shots of birds of prey. They always fly away when I cautiously try to approach them...