what sort of task is it to set up a cluster of say 4 PC with different specs?

At work there is 4 of us in a group and we do a lot of debugging, this involves editing and recompiling code. It occured to me that connecting all the computers together to combine the processing power would be a great advantage. and since we aren't all trying to compile at once this would also be efficent. how easy is it to set up a cluster of 4 desktops still using each as a workstation. the options would be either - connect them all together running one OS over the 4 and providing a login from any machine, or running them as individual OS's and using either a special compiler to make use of the other PCs or a VM that would run over the cluster.

what is the general idea in this?

Well, that wouldn't be a Beowulf cluster then. Most real clusters are single-purpose systems, designed to do massive amounts of computations and basically nothing else; you wouldn't use each node as a desktop, they wouldn't even have desktop software installed (or even hardware, for that matter).

Connecting regular desktop machines together as nodes of a cluster is something that was worked on by the openMosix project, but unfortunately they are no longer active.

If you just want to use the combined power of the machines for compilation, you might want to look into distcc. distcc is a system for distributed compilation of C/C++. Basically it sends out files to be compiled to slave machines, they send back the binary objects, and then it combines them all up at the end. It is simple and can be very effective. You can install distcc on basically any distribution, and you don't need to change anything else on the machine to use it.