If you select expert install mode it will give you control over everything.
However, its best just to create free space and let the installer create partitions to fill it.
How did you create the fourth partition? Is it a linux or DOS partition? linux has its on partitin type and the min install is really two partitions. / (root) and swap. Depending on how much RAM you have some consider swap optional now days.
Here is the online documentation for Mandrake.