I looked into this and the 4 year is a recomendation but every company I looked at says at least every 5 years, and maybe every 2 years if you work in the sun. The interior suspension may only be good for a year.
http://www.nifc.gov/wfstar/hottopics/hard_hat.html
http://media.msanet.com/NA/USA/Head...rdProtectiveCapsandHats/816803_01_03.pdf
http://12.181.169.139/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=HDP

Greg Fretwell