The Tier 4 emission requirement is primarily for the equipment manufacturer. That is, by a specific date their new equipment production must be compliant with the Tier 4 standards. The end user does not have to convert their fleet to be compliant. The end user is responsible for the work environment and must comply with air concentration levels of of CO, NoX, HC, etc. Tier 4 lift trucks help the end user comply along with the in plant air exchange system.
Lift truck engine emission standards started in 2000 with Tier 1, then Tier 2 , 3 & now 4. Each phase getting tougher to comply with. & more costly.