The "<" being present tells us that the equipment / test can not measure below that limit - i.e. <0.06 is the lowest they can give you. When you got a <0.04, it was probably a different test or different test equipment.
Both mean "undetectable" in the context of the lab and test.
That is why we have so many running discussions about staying with the same lab, and checking the equipment used, if you can get that information.
Your <0.06 and <0.04 could in theory be for the same actual value - let's say your real PSA is 0.03. Both are correct to the limits of the test.