Joe,

I'd set the distance based on the nearest surround speaker (4 feet, in your case). This way, a longer delay will be applied to the surrounds than if you had entered 6 feet or 8 feet. Too short a delay for the surrounds means that there is a chance you'll hear the sound from them before you hear the sound from your main speakers (i.e., the Haas effect). This is more likely to happen if your main speakers are further away than your nearest surrounds. Delaying for 4 feet probably won't make a big difference but, since you can only enter one speaker distance, no harm in compromising on the side of caution.

Best,
Sanjay