In Europe, the emission standards are called Stage, whereas in America, Japan and Korea, Tier is used. However, the two terms are comparable.
 
Tiers 3, 4i and 4f are the American equivalents of the European Stages IIIA, IIIB and IV.
 
stage tier 

Europe America, Japan en Korea
Stage IIIA Tier 3
Stage IIIB Tier 4i
Stage IV Tier 4f