Basic math.
Comparing the number 4 to the number 6.
It is correct to say that the number 6 is 50% bigger then the number 4.
It is also correct to say that the number 4 is 66.66666% of the number 6, or 33.333333% smaller then the number 6.
It is NOT correct to say that the number 6 is 33% more then 4, it is also not correct to say that the number 4 is 50% less then the number 6.
Both percentages are correct, 33.3333 and 50%, IF you state them correctly.
6 cores is a 50% upgrade over 4. 4 cores is a 33% downgrade over 6.