Does splitting a process across 4 terminal windows decrease the time it takes?
What you describe (split job into parts, total CPU usage stays the same) indicates that your task is (or gets) I/O (or perhaps RAM) bound. Try splitting among several machines, not just several processes on the same machine. If the program you are using comes from somewhere else, see if there are newer versions, bug reports on performance, or perhaps tricks and suggestions, on its webpage or user sites.

Use the tools on your operating system to see what the bottleneck really is. As you don't say what operating system this is, I can't give concrete suggestions. Perhaps look into  uxix.stackexchange.com, superuser.com, or serverfault.com (all part of the stackexhange network).