i would probably judge then not by the length of the snake but its body mass, the snake should still be quite small but could be pretty long i would say the fatest a 4 month old that ive seen would be about like the tip of my finger and i have tiny fingers... you could also tell by if the snake looks like it could be eating a hopper or a pinky or a fuzzy, a 4 month old should *i could be wrong* but only be able to safely eat a pinky.