I would have to agree with dbrinkmann99. As far as i can remember from my vcp courses, the cpu scheduler eventually needs to allocate the work to a physical core. A quad core vm is being allocated with all it's 4 cores to the physical cpu, also with light work. If your host is running on only 4 physical cores, you can get some nasty cpu ready time as soon as other vm's start to get busy because they can't get cpu time while the 4 core vm is being scheduled.
And thats also what i see on some of my esx hosts with 8 physical cores and 30 vm's. As soon as the quad core vm's start to get busy, cpu ready time gets over 10%. In my future environment i would not save money on cpu cores.