I'd steer clear of tier 4 unless you prepared for the problems they have and the care needed to keep said problems from happening. DEF systems are picky in more than one aspect. Storage of def is important. Bulk storage is not recommended for small fleets due to mishandling and improper storage. Sunlight harms def. Do not leave def in direct sunlight. Most manufacture understand this and put product in opaque containers. Misuse. 1 capful of DEF introduced into the fuel system can cost thousands in repair. It attacks aluminum fairly violently. Most fuel systems especially common rail have alot of aluminum going on I.E. fuel rails. It's very easy to introduce DEF into the fuel system and just as easy for a dealer to recognize the contaminate and halt the warranty for repairs. Dirt sawdust anything introduced to the system can destroy the pump and doser valves if introduced and the filter doesn't catch it. That being said I try to stay clear of tier 4 albeit its becoming increasingly difficult to do so. It's poison for the diesel engine. Much like EGRs and pretty much anything the government straps to the side of a diesel engine in the name of "environmentally friendly"
