How do you know?
I'm curious what you mean by down trim. Are you using the position of the tab relative to the elevator to establish your neutral point? Remember that the function of the tail is to press down on the back of the see-saw and raise the nose. I admit there is more drag if the tab is deflected in either direction, but the biggy might be the amount of down force on the tail producing induced drag. So, maybe nose-down trim is less drag? I'm not familiar with the -4. Can you shim the angle of the HS? This issue may have applicability to the other models, too.
