I hope it would be more than just a 4th, as the report on ESPN claimed "Buffalo would only trade the receiver if it was able to get "something substantial" in return." link I hope they're not viewing a 4th round pick as substantial.
There's also more than what is coming back from the other team involved in the trade. There are also opportunity costs since you can only keep so many WRs. So not trading Evans will cost you a WR on your roster currently that you value, likely 2 of Easley, Roosevelt & Davis. So add their value into the return.
Finally, why cherry pick 2008? Is 2-3 players the norm for a 4th round pick? 2006 garnered 15+ players including Eric Smith, Owen Daniels, Brad Smith, Jahri Evans, Stephen Tulloch, Leon Washington, Stephen Gostkowski, Brandon Marshall, Damato Peko, Barry Cofield, Elvis Dumervil, Ray Edwards, Ryan Sims & Willie Colon among others. Not that this list is also typical, but let's not pick a bad year and call it the norm.