There's no way a 106g snake puts down a small rat. I think your scale is messed up. Make sure your scale is on a flat level surface, every time you go to weigh put a nickel on your scale first and it should read 5 grams.
If your snake was really 106g at 4 months that is fine, age has nothing to do with it. I have a 8 month old that weighs 200g and came from a reputable breeder and she eats the correct size meal and has not missed a meal in the 3 or so months I've owned her and according to the breeder hasn't missed a meal for him except her very first.
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