You've got a great result. With your case history (low pre-surgery PSA, neg DRE, 3+4, negative margins, no SVI or LNI), most doctors would tell you that you have no need for a post-surgery ultra-sensitive PSA test.
Based on what you wrote here, your latest test has a low-end measurable threshold reporting limit of 0.1 ng/mL, and your result was below the lowest threshold.
There are differences in measurement techniques, so the advice doctors give is to have your results measured consistently at the same lab...at least from now on.
The comment about the reference value of 4 ng/mL is most likely placed there to show that the standard range for "passing" the test is less than or equal to 4 ng/mL. While this is the "standard range," after RP, your goal is to be undetectable...and you are!