Fullsize HDMI vs microHDMI, electrical considerations
If only the difference of connector is considered, EMI would be approximately the same. There is very little difference in the pinout in general, and there is no difference in the high speed TMDS lanes which are the most likely to cause EMI. It is just a smaller connector mechanically, no other electrical difference. Most likely there will be more difference in EMI due to changing Pi 3 to Pi 4.