Unfortunately you /boot partition must still use ext3! There is no support with the minimal install of centos 6.3 with grub bootloader version 0.97 for ext4. Buyer beware!
Do you need to read or open a .IMG file created with DD backup on Windows? The best tool for the job is called DiskInternals Linux Reader. This multi-platform program supports ex2/ext3/ext4/hfs/reiserfs filesystems.