Hard to tell by plastic, as people update it all the time.
To be sure, you need the first 3 digits of the VIN stamped on the steering head. Compare them to the table at this link:
http://www.merrittmotorcyclesalvage.com/yamahaidentification.htm
Looks like if it is 5GR, it is a '99 YZF400. 5JR = 2000 or 2001 YZF426, 5SR = 2002 YZF426
Looks like first year of production for the 400 was 1999. The 426 was from 2000 - 2002, and the 450 came out in 2003.
If you don't have access to the bike to view the first 3 digits of the VIN, have the seller check them and verify with you by email or phone and compare to the table.