That problem is complicated to fix. I've managed to fix in Iphone 3, but have not tried on IPhone 4. I will try to explain it:
We start from 0, recently on the computer.
1. You add the host where you have indicated above.
2.Abres itunes and install the same version you had, remember control + shift
3.Though error you successfully installed, you open a program called Timumbrella.
4.Seleccionas your device and you give Fix recovery
5.Se reset and ready, like the first day.
If you have any questions, send me an privadomy hacer.Porque can see that another solution is that I Mandaras and could fix me, but would do what I have explained to you, do.
Good luck and I will fix reading a lot on forums and tutorials.