It's actually FIVE chickens now but here's a link to the ordinances (pdf):
http://www.salemchickens.com/summary_of_salems_ordinance.pdf
And a printable "chicken keeping permit":
http://cityofsalem.net/Departments/CommunityDevelopment/BAS/Documents/Chicken%20License%20Application.pdf
Update: as of 3/19/12 we can now have FIVE hens in Salem, there will no longer be yearly inspections and our coops can now be closer to our houses and too our fences/neighbors too.
Interesting thing though...The city of Salem is in two counties the ordinances apply only to Marion County...