There are a number of reasons why it could be slower,
l1,l2, and l3 cache size - if the dual core can keep more stuff in cache then it doesn’t have to wait on slower memory. If the cache is the same but split among four processors then effectively each processor is getting half as much cache as previously.
Smaller amount of main ram - maybe it is having to hit swap more on our quad core computer.
Different build configuration - your build used on your dual core could be optimized.
As others say - number of tiles - tiles divide up the scene into number of pieces for each render, the more tiles the better the load can be distributed between the processors. A small number of tiles and towards the end of the render the other processors will be doing nothing.