Reading a review of the 4x4 it seems the IR receiver is different from what is included with the trains. From the review all the information we got was "improved CMOS motor driver with lower power loss". Can someone expand on that?
I have done an extensive review of both receivers, see it here. This other page shows the performance of various motors connected to both receivers. Note that because of a problem in Power Functions M-motor, the new receiver is not suitable for driving several of them on the same output. See this LEGO official notice.