Carbonfiberboy 

I think 4cm change would imply really big feet. You say "get away with." Why do you want to change your saddle height? Which comes first, chicken or egg, heel-down or saddle height? What's the objective?

When I ride, I concentrate on the feel of my heels throughout the pedal stroke. I like to constantly feel the contact of my heel with the heel cup of my shoe. Thus I relax my ankles to promote that. However, trying to relax one's ankles doesn't mean that they don't do anything. Our bipedal heritage insures that we automatically contract our calves to maintain pressure on the pedals when we push down with our legs.

At the top of the stroke I try to lift my toes so that it seems like I'm going over the top of the stroke with heels down although video will show they aren't really "down" just at a more acute angle w/r to my lower leg. Then I push forward, but more with my heel than my toe and on the downstroke continue to maintain contact with the heel cup. Then at the bottom of the stroke, I pull back with the heel cup, then on the backstroke attempt to lift my leg some but again maintaining contact with the heel cup, the relaxed ankles allowing the toe to drop until I need to flex it up again at 11 o'clock.

So that's how one pedals "heels down." To set saddle height, simply use the heel-on-pedal method, which is described on many fitting websites. Google and look at a few different explanations. After you use that method to set saddle height, you may need to play around with it a little, up to 5mm one way or the other, to get what feels like good power during the pull-back phase at the bottom.