that you want to use that 1TB external drive partitioned into 4 partitions, each with XP installed, and then each partition as a boot drive for each of the 4 computers?
If so, that is not possible. There are plenty of articles explaining why on a Google search, How to boot XP from an external drive.
I have 4 desktops running XP and connected to the same broadband router. I have a 2 TB external hard drive and a 1 TB hard external drive and each of the desktops have 40 GB hard drives except one has a 40 GB and a 100 GB hard drive. I was wondering if I could create 5 partitions on the 1 TB hard drive and use 4 of the partitions as C: drives for each of the desktops and the 5th partition as extra storage drive space? In other words the 4 partitions would each be a C: drive for each of the 4 desktops. The 40 GB hard drives on each desktop would also be used as extra drive space. Currently I have the home network setup and the 2 TB is connected to one desktop and the 1 TB is connected to another and I have the external drives shared over the network. Isn't there free virtual machine software available that can do what I want?