plasticity index, by definition is performed on the minus 40 sieve material. The lab is required to report NP if the sample is non-plastic. The ASTM does not acknowledge some range, like non-plastic to 4 as a range. Not saying that wasn't the lab's intent, it's just not a standard reporting method.
I'd ask the lab. You have no idea how few folks actually read the ASTM methods for how to run the tests and what/how to report the test results.
I WAS ASSUMING THE LAB WORK WAS DONE USING ASTM STANDARDS. YOUR ORIGINAL POST WAS NOT CLEAR THAT THE TESTING WAS PERFORMED USING USDA-NRCS STANDARDS, WHICH I HAVE NEVER EVEN SEEN (BEING AN ENGINEER AND SUCH).