I'm not to sure but at 4cm it's still virtually a sling so adult coloration may still take a while (also means a great difficulty in accurate sexing), however it shouldn't matter what sex it is as I understand male only lose their blue after the ultimate molt
It sounds like your sling is either to young for its color yet, or you have a male. Have you personally sexed from a molt? Or were you told that? I have a F H. Lividum who is almost black unless you look at her in direct light, but after she molts she is bright blue again.
You get the blue variant and you get the darker variant of the lividum!
But id say yours is too small to tal unless you know which it came from!
and even the more blue one is not that blue, unless taken in the sun
or under flash! male female doesnt have anything to do with colour at this size