At 4 inches it's pretty hard to tell. Once sexually mature, the toetips change colour and that is usually the age you can be sure of it's gender. Since it's a lucy, it's toes will turn darker. You can tell by the size of the cloaca, if it's very small it's a female and if it's larger it's a male.
On a side note, when my axies were maturing my dad saw the size of Peri's cloaca and freaked out because he thought it was prolapse but he calmed down after I told him it just meant she was in fact a male.
Judging by your axie's cloaca size it looks like a male, but I'm not completely certain.