[help] Trouble installing BT4 in dual boot.

Hi guys, sorry for the really newb question but there it is : I followed the how to, step by step etc. so no mistake. The problem comes from the partitionning and I can't figure out how to install it and the desired partition. My HD has 4 primary partition, 3 NTFS (recovery, system, files) and the fourth... well anything, there is nothing on it. I tried formatting it to different formats, nothing chges, even unformatted...
Any idea how to get BT installed ?
Thanks in advance :)

[here are some snapshots if it may help : http://img713.imageshack.us/img713/4782/snapshot2g.png, Imageshack - snapshot1yk.png,http://img28.imageshack.us/img28/4130/snapshot3w.png]

I think your best option would be "Guided - use the largest continuous free space". It will use those 29% of free space you have on the drive.

BTW, you only have 3 partitions, the 4th you mentioned is just unallocated space, so it's not considered a partition.

Well I know it is not considered as a partition, but I also tried with it formatted, to many formats (FAT32, NTFS, ext2) and it changed nothin but suppressing the "guided option", wich only propose me to use the whole disk...
And is says "freed" place, so i believe it means the place gained from changing sda3' size, right ? and not from another partition, or from the unformatted partition ?
I'll try again anyway, and thanks for answering ;)