What is the file extension of the file that is loaded in a Xilinx FPGA?
This file is commonly called a "bitstream". Xilinx devices' bitstreams have the extension '.bit' and are generated by a program called 'bitgen'. '.bit' files are binaries and those are generated by default; if you want the ASCII representation of the bitstream, run

bitgen -b &lt;your design&gt;.ncd


and then a '.rbt' file will be generated in addition to the '.bit' file.

For further information about generating Xilinx bistreams, see Chapter 15 in this user guide for ISE version 12.4:

http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_4/devref.pdf

For more information about Virtex 4 bistreams specifically, see the configuration guide for that device:

http://www.xilinx.com/support/documentation/user_guides/ug071.pdf