4 years across all python species is not accurate and its wrong to make such assumptions by a keeper or a vet. The thing is, not a lot of people are experts in the topic of Boelen's. The co-author of "The complete carpet python" who wrote this book with Nick Mutton, Justin Julander, is one of the leading experts and has spent time studying this species in the wild.
All that aside, another species that little is done with in captivity is the Timor Python, a favorite of mine. I only mention this because many people tried to breed this snake and have failed or given up. Why? Because they made the assumption that they are breed ready by 4 years of age. The correct answer seems to lean toward 7 years for Timors. Boelen's are likely to be the same. Due to their cooler climate, they do not metabolize as quickly as other species. This is crucial to understanding the species. This slower metabolism means less meals consumed in the wild, and slower growth as well as a longer period till they are ready to breed. I hope this gives a little more information to answer your question.
Do not force an increase in metabolism by increasing heat - you will likely find that the snake stays on the cooler end of the cage. Again, just my two cents.
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