If your CPU supports frequency scaling as a power saving feature, you may be able to use that to slow it down.

Another option is running another process which hogs the CPU; if you alternate between performing some CPU-intensive operation and yielding control to other processes, you should be able to fiddle with how much time is spent doing each to get the other process to consume the amount of CPU you would like, and if you make sure the hogging process has a higher priority than the process being tested, that should approximate a slower CPU.

A third option is running it on the high-speed machine and dividing all the results by 4 to simulate a machine 1/4 the speed. :)