We have the oriental type of Bengals (long slim bodies that aren't very muscular) and we also have the more chunky type of Bengal.
I have not noticed any difference in weight or size after the cats and kittens have been neutered. (bare in mind that once we neuter, they aren't with us very long as they are supposed to be re-homed)
I have actually noticed that very close to 2 years old is when my slim girls start to bulk out, keeping their long bodies but gaining more muscle, increasing in height and looking much more healthy. These girls are actually entire stil,l but up to the age of 2, I have no chance of showing them because body wise they are too small and are no match for the boys or the Bengal standard.
When I had my silver, she did actually grow quite a bit when she had been spayed....... she was over a year old when she was spayed so we have no idea if this was to do with the spaying or if it was genetic, but after seeing the age that they stop growing, I think size is genetic.
Some are small but catch up later and some are whoppers at birth that don't amount to much compared to others in the litter.
As long as your kitten is eating well, sleeping well, drinking and getting exercise, I don't see any reason for you to worry. 4kg is plenty big enough, especially at less than a year old as their muscles haven't developed yet.