How To Sex A Kitten At Four Weeks?

i don’t know what to do, i already tried looking it up and found that there’s a lot of different opinions about this.

the kitten is almost 4 weeks old and we got her from the shelter two days ago…i know she’s already having heat cycles but we haven’t seen any blood yet…anyway, the vet at the shelter told me (he said he has done this before) that sometimes you can tell by how they walk around or whether they move their legs when running….and right now, she walks all weird-like because its like her back legs are kind of weak. so i just want to know if anyone has ever used this as a way to find out how far along your cat is?