Hi there!
Not an expert, but I did a quick search and found six listings of an F SI2 radiant with prices ranging from $3963 to $4062. (I'm assuming same stone with multiple listings, since specs were identical.)
I hope that helps! Perhaps the experts can add their thoughts.
If you could add a bit more info it would be very helpful. Quality of finish/polish, girdle thickness, depth and table %'s, and Sarin Info if it is available would be great! You can do that by editing your previous post. Without more info, it's hard to give you a good idea of a fair price.
Have you seen the diamond in person? If not, I'd suggest getting a copy of the Cert and inclusions plot to take a look at. Find out about the size, type, color, and location of inclusions. SI2's can be especially tricky to find an eye clean one.