From this source published in October of 2018: http://www.servicetruckmagazine.com/features/tier-5-emissions-standards-not-expected-anytime-soon/So you know I'm not blowing smoke...lol
From Heavy Equipment magazine....
The scope of Stage V in Europe, however, will differ somewhat from all the regulations that came before it, says Payne. Previous regs exempted engines under 24 horsepower. Stage 5 will likely cover all engines regardless of horsepower. The rules for flex credits and transitional engines, which gave some manufacturers breathing room in the old regulations, would likely be restricted in a Stage V scenario, Payne says.
It will be quiet some time before we see any Tier 5 here. Remember that the current system was started in 1992 and took until like 2016(?) to get fully implemented.
With the current administration and the amount of push back/lobbying that manufacturers, large contractors, not to mention Railroads will do I highly doubt this gets put on the books and implemented in the next decade.
Also I don't disagree with the Tier 4 requirements entirely anyway. I don't like the need to put DPF in Trucks along with Diesel that is just another way to nickel and dime people but without the industry getting a push they will have no reason to improve on the emissions front, and instead just keep fighting on cramming the most horsepower into things as they can.
These pushes help us get more MPG saving us money (not including the cost of DPF which I already said I dislike), helps future generations by hopefully leaving this Earth cleaner than how we found it, and drives innovation to improve manufacturing processes. I think the biggest deal we all have is paying the bill for it and not being able to work on it ourselves.
The latter issue is the manufacturers wanting to force OEM repairs to increase their pocketbooks and blaming government entities for complex components that they don't trust us to mess with.