The 08+ are better because of additional adjustments, there are slight valving and spring changes in the earlier models but IMO nothing I'd get too hung up on. If the 04 shocks are still in good shape and aren't already worn out and in need of rebuild, then I'd certainly look into them.
I ran 04 yfz fronts on my +2 front end for a couple years and was happy with the results.
Yes, but I guess I kind of consider the 09+ 450R and '10 YFZX as different atvs. The YFZR uses 18.25" or so shocks, the YFZX uses 16.5" or so shocks, neither of which will work with standard travel 660 a-arms.
look at the profile of the rezi's...04-07 shocks are the same except for the 07 special edition... the 07 se had both hi and low speed adjustments... then in 08 all the shocks were the same.. now there is a 09 yfz (not the "r") and they will work on the raptor as well..
The "old" yfz450 used a 15.5" long shock, which is basically the same length (within a 1/4 inch) as the 700R and 660R shocks depending on who is measuring it. Yes it did lower the front end quite a bit with the +2 a-arms on my 660. I didn't try that setup on a 700.
The YFZX shocks are a little too long, they're the same length as the old Honda/Kawasaki/Suzuki 400's and the Honda and Kawi 450's, which is about 16.5" I'd expect they'd bolt up, but I can't tell you if you're going to run into any binding issues with the ball joints.
Oldstyle, then yes I'd pull the yfz's shocks for your setup. I can't think of anything else off hand that you'd need as long as your bike is in good shape.