I cant explain it to well..but a 3rd edition modual translated to 4th will have the PCs fight small groups of enemies that wont be balanced out to well without adding more monsters.
A 4th edition modual translated into 3rd will cause the PCs to fight a giant amount of weakings with no challenge involved or a giant amount of tough enemys that will cause a TPK.
And let me take a moment to thank the Paizo design team for their work, and for their answers on this thread. I recently picked up both Crown of the Kobold King and Rise of the Runelords, and I have to admit that I am exceedingly happy with both products. Good work!