Where you can learn more?
Conduct simple internet search on EPA Tier 4 Emission Standards. There is plenty of material to help get you grounded in EPA Tier 4. The following links should help get you started:
EPA Regulations for Non-road Diesel Equipment
The Diesel Forum
General info on off-road diesel engines, equipment, retrofitting, etc.
Association of Equipment Manufacturers
Bobcat Website - Tier 4 Content
Diesel fuel standards and timeframes