Have a look at this:http://uk.pcpartpicker.com/p/35U8J
It comes to just under £700. But I haven't chosen a hard drive or OS.
You'll want to get an SSD as they are much faster and more reliable. But are more expensive (Sorry for going over).
That setup will easily be able to run BF4 on ultra at decent fps.
I chose the i5 4670k because I thought you might want to overclock, and an decent air cooler if you do.