This winter I set up a Light Rail 3.5, it was well reviewed and reasonably priced, widely sold.
"I was going to put 2 x 600 in cool tube on a light mover over the same canopy as before."
I have a Sun System 2 air cooled reflector, I unhooked the fan ducting so the motor would run smoother on the rail. I was worried that the drag of the ducting would burn out the motor, but I've read of people putting up curtain railing next to their Light Rail and hang the ducting from the curtain rail. I may do that later but I have no heat problem now.
If you're going to run ducting you may want the commercial heavy duty Light Rail 5. You can get extensions to run more than one rail/light on one motor.
I had a 4.5' x 4.5' grow area with a stationary 1000W HID and changed to a 8' x 3' with a 600W on the Light Rail. 3 square feet more space and the light is focused on a more even area, intense light everywhere - 50W per square foot for half the light cycle.
The light is over each half of the 8' x 3' garden 50% of the time, Subcool says that he grows on a (6 on / 18 off) cycle with great results.
"Now how much less will my yield be compared to 4x600 ?"
You may get less yield. Some people say 75% but I haven't noticed any reduction, an increase if anything. If you can run CO2 you can bring your yield back up, or increase it.
http://www.canadianwholesalehydroponics.com/products.cfm?cat_id=99&show_products=1&catpage=3
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http://www.lightrail3.com/garden/LR35.php
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