...Ok, well, that might be because your disk is fully partitioned already (?). I don't know what logic the installer uses to give its choices.
Still, if you can get to the manual partitions part then that's a good start. What you'll probably want to do is to choose that, then select the partition you want to install to (you said you have a swap partition, a 'just there' partition, and a fourth partition that you want to use, right? choose the fourth partition), choose 'Change', and then set its type to ext4 and its Mount Point to '/'. That identifies it as the root partition.
(In this case I'm using the guide at http://netgator.blogspot.com/2010/10/ubuntu-1010-installation-guide-and.html as a reference point. If your installer doesn't look like that, and/or if selecting specify partitions manually doesn't get you to a screen like this, then I'd probably need to know what Ubuntu version and media you're using for your install...)
no subject
Still, if you can get to the manual partitions part then that's a good start. What you'll probably want to do is to choose that, then select the partition you want to install to (you said you have a swap partition, a 'just there' partition, and a fourth partition that you want to use, right? choose the fourth partition), choose 'Change', and then set its type to ext4 and its Mount Point to '/'. That identifies it as the root partition.
(In this case I'm using the guide at http://netgator.blogspot.com/2010/10/ubuntu-1010-installation-guide-and.html as a reference point. If your installer doesn't look like that, and/or if selecting specify partitions manually doesn't get you to a screen like this, then I'd probably need to know what Ubuntu version and media you're using for your install...)