I have to step in here because it is true that a hdd can have a maximum of 4 primaries but is is also true any one of the primary can be turned into an extended partition.
If a Linux user starts partitioning a hard disk and opts for a logical partition the hda1 (or sda1) will immediately turn into an extended partition and his/her first logical partition is always at the 5th position, namelt hda5. Hda1 to hda4 are reserved primaries but a maximum of one can be an extended partition.
Don't take my owrd for it, try it with cfdisk program on any hard disk. No permanent change to the partition table unless you click "write".
If you want to see an example here is one