You don't need to worry about this.
If you have four clones of a reentrant VI and they all run at the same time with significant CPU use, LabVIEW will automatically use all available cores. Of course there are also other processes that need CPU (e.g. the UI thread and anything else running on the PC), so it is impossible to fully dedicate any one core to any specific subVI anyway.
What do the VIs do? What is your concern?