Yes Sandy, the main consideration seems to be that he's saved them time and money by admitting the murder, so he's rewarded by getting a 4 year sentence. (9 = 4 in reality).
// and after little more than four years may be free again into society. //
Yes, but the people determining the sentence have obviously taken into account the fact that he's the type of person that never go anywhere near their neighbourhood, so from that point of view he's very safe.
// finally admitted that he and only he was involved in Kacey's tragic death. //
Yes divebuddy, I take that to mean that for a while he was trying to blame the mother, at least partly, but eventually came clean.
As I said, he seems to have been rewarded for that, when in fact it should have had the opposite effect, as trying to lay the blame on someone else makes his behaviour even more despicable in my opinion.
I don't know what the sentencing guidelines are for this kind of crime. Its manslaughter, not murder and he has been given 9 years for it. The sentencing would have been up to the Judge after all.
By the way, there is no guarantee that he will only serve 4 years out of his 9. Whether he gets time off his sentence will depend on how he behaves in prison. All that I have seen seems to suggest that he has a anger-management problem, so who knows ?