The National Institute of Justice (NIJ) has modified the 01.01.03 ballistic standard for the testing of Body Armor. In general, the new 01.01.04 standard is almost identical, but just slightly tougher. The interim 05 standard is simply an assurance that no Zylon is used in the manufacture of 04 Certified vests.

The 9 mm FMJ test rounds are exactly the same, it is the secondary test round has been slightly changed...

To keep blunt trauma under the 1.7" (44 mm.) maximum, an 01.01.04 vest might be 1 to 4 layers thicker depending on the over-protection previously built into the vest. More is not necessarily better, in fact we often recommend Level III-A in the 01.01.03 Standard to keep the vest slightly more flexible and comfortable.

The NIJ says:

"NIJ Standard-0101.04 is not intended to restrict or otherwise influence the procurement and use of NIJ Standard-0101.03 compliant body armors... ... this revision for new model compliance testing does not invalidate or render unsuitable any body armor models previously determined to be compliant using NIJ Standard-0101.03 requirements." - NLECTC.org

So, on either standard, you can rest assured you are getting a high-quality vest. There are MUCH more important factors to consider – such as how well the vest covers your body, and how comfortable it is.