4 lights ir 5 in my room...please help!

Ok, my room dimensions are 7'8" tall x 7'4" wide x 19'7" long. I have 5 large adjust-a-wings but only have 4 installed in the room.
According to the adjust-a-wing info included with the wing and from talking with sales people 4 is all I need in the room.
These all have 1000watt HPS bulbs and ballast.

#1 Should I just use the 5th wing and ballast?

#2 Will the added electricity expense be justified by increased yeild?

#3 Would the 5th light add to my light cossover created by the additional light?

#4 Is what I heard from the adjust-a-wing people just a sales pitch?

As always, thanks for the great support!

I would add the 5th light, with 4, each light has to cover about 5x7 foot area,most agree that a 1000 watt light is good for up to a 5x5 area.
that said a light mover might give as good results as a 5th light w/out drawing as much power.

Light mover is not really an option fo me. Do you think that the claims at adust-a-wings is overrated? I was tol by a guy at a local shop that 1000watters are good for a 4'x4' area. 5'x5' is the max. Anything beyond that should require an additional light.