Whith both units sweeping and placed horn to horn they needed to be more than 12ft apart to not cause any interference during normal operation.
With both units sweeping and placed side by side they needed to be 4ft apart to not notice any occasional interference during normal operation.
Again, this was with 1 sample of an M4 and 2 samples of an SE.
If your M4 or SE is more or less sensitive than mine, your results may differ.
One think to note, it you pressed the menu button on the SE, the unit will stop sweeping and cause the M4 to alert in the neighborhood of 35.45 even at the distances noted above because it stops sweeping.
The M4 does not stop sweeping when enterning the menu mode so it does not create the issue to the SE that the SE creates to the M4 during the menu function.