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:
The Diesel Forum
www.dieselforum.org
General info on off-road diesel engines, equipment, retrofitting, etc.
www.clean-diesel.org
Association of Equipment Manufacturers
www.aem.org/SRT/Regulatory/Tier4/
Bobcat Website - Tier 4 Content
www.bobcat.com/excavators/tier4
Catepillar websites
www.cat.com/technology/acert-technology
Cummins Website
www.tier4.info