Woohooo!
Yes, it is a Sigma f/4.5 and it was used, thus I saved $3,500.00 Vs. a New Canon f/4 @ $ 5,499.95.
The f/4.5 also makes it a good deal lighter than the Canon f/4 (but thats a small plus given once you are in this category,. what difference is a pound here and there? )
I purchesed it used from KEH and I have to tell you I am stunned! 
The customer service/sales was excellent,. the salesman took the time to look up any questions I had that he could not answer,. which were few as he pretty much had answers to most of my questions. All told he spent about 20 minutes on the phone with me before I was convinced and coughed up my Card number 
I have a 14 day no hassle return period, 60 day KEH warranty and I went for the full year MACK warranty for an additional $50.00
BUT I WON"T NEED IT!
This lens is brand new! There is not a single mark on it. The front element (which is in fact replaceable on this lens) Was PERFECT,. (not even a grain of dust? ) the body is perfect,.
...THERE ARE NOT EVEN ANY SCUFF MARKS ON THE TRIPOD MOUNT/THREADS!!!
This lens looks as if it was never out of the excellent soft case that comes with it!
A quick test on a tripod in my kitchen and I am convinced it is the fastes most accurate focusing lens I have (including my Canon 17-40 f/4 L )
O-kay,. so I am very happy

And,. to top it off,. it is a nice sunny day!
See ya! 

