While this doesn't address tier 4, this might be some help:
From the GE website:
"Tier 3 is a continuation of the US Environmental Protection Agency’s program to monitor and lower exhaust emissions by locomotives. It follows, naturally, Tier 0, 1 and 2 emissions standards, and marks a 50 % reduction in acceptable particulate matter levels from Tier 2 levels.
The process of enhancing locomotive emissions standards began with Tier 0: all locomotives built after 1972, be they new or overhauled, must be up to those standards. Then, from 2002-2004, all new locomotives were built to a Tier 1 standard. Locomotives built between 2005 and December 31, 2011 were held to Tier 2 standards—and when it comes time to overhaul those locomotives, they will be brought to the most regulatory level.
The new regulations will be implemented in two phases: on new locomotives built starting in 2012; and starting this year, via overhauls to locomotives that were originally built between 1973 and 2004. Tier 3 locomotives will emit no more than 0.1 grams of particulate matter (PM)—tiny particles that escape from the exhaust after the combustion process—per horsepower hour. Furthermore, they will have maximum nitrogen of oxides (NOx) levels of 5.5 g/bhp·hr.
So how do they do it? There are various aspects to the enhancement process—the exact process depends on factors such as the type of locomotive, its age, etc. But ultimately, improvements are made in four areas: the turbo charger, which controls the volume and temperature of the air that goes into the engine; the cam shaft, which controls the timing of fuel injection and the opening and closing of the engine valves; the injectors, which determine how much fuel is injected; and the ring pack, which help keep the oil used to lubricate the pistons from entering the combustion chamber.
When a locomotive comes in for an overhaul—which takes place, on average, every seven-to-10 years—the railroad or GE removes the engine from the locomotive, then ships the engine off to GE’s manufacturing facility in Grove City, PA. There, the GE team rebuilds the original engine to one that is completely upgraded to the new regulatory requirements, which then becomes part of the pool that will be used to replace the next engine scheduled for overhaul. That way, locomotives are back on the rails after minimum downtime, instead of remaining out of operation for weeks while GE rebuilds the engine. All that’s left from there is to upgrade the locomotive’s software, which can be taken care of by either GE or the railroad, and the locomotive is good to go.
As a part of the Tier 3 program for new locomotives built after 2005, GE has begun to implement a high-pressure common rail fuel system. With this advanced system, high pressure fuel is available at all times, allowing for more control over how much, how long and when fuel can be injected. With more control of the combustion event, the locomotive is able to produce fewer emissions, meet EPA standards and, most importantly, reduce their impact on the environment."
Anyway, as far as it goes now with Tier 3:
particulate matter is now reduced to 50% of the previous Tier 2 level, tp 0.1 grams per horsepower hour.
similarly, nitrogen oxides are reduced to 5.5 grams per horsepower hour.
Tier 4 will require a greater reduction in these figures, certainly in nitrogen oxides, and this is to be acheived by burning urea in the exhaust stream over a catalyst which will give the lower levels required. Particulate filters have been mentioned, too.
Note that these standards do not directly address carbon dioxide emissions except by reducing fuel consumption.