A 1000w hps will provide good lighting for a 4x4 area, if you put them on light movers you should be able to cover a 4x8 area with each light.
How many you will need depends, as you say, on how much space the plants take up. I take it you're looking at doing a SOG style grow? Lots of fairly small plants, creating a Sea Of Green, so you may get 2-3 per sq ft, assuming you lollipop them and keep them fairly close together.
Imo it's not a good idea to run an op this size with autoflowering plants, the seeds are going to cost you a fortune and you'll have no control over how big they get, but it's your grow.
Also that amount of light will require a serious amount of ventilation. With a room full of HID lights the rule is to have a fan capable of extracting the volume of the room 2-3 times a minute. In your case, depending on the height of the ceiling, you'll have between 3,600 and 4,500 cubic feet. It will take several large fans to effectively ventilate that space, and you'll definately want to be cooling your lights.
Get a good read of the FAQ, you should know from memory every bit of info, and be able to diagnose plant problems by sight before you start doing a grow on this scale, there is so much potential for things to get out of hand if you don't know exactly what you're doing.