I already went through this on the other thread. There could be myriad causes ...
1) Was she embalmed (even if not completely, perhaps partially) whether with or without the knowledge of the other sisters there?
2) Was she on antibiotics or other medications before she passed away?
3) Was the soil of lower acidity?
4) Was the coffin made of cedar wood, which repels insects?
5) We don't know how long the coffin had been broken open.
6) Did the inner fabric lining of the coffin wisk moisture away from the body, allowing it to dry out and mummify?
Only 4 years had passed, and rate of decay can vary due to many different factors, normally requiring up to 10 years, but embalmed bodies or those subject to various other conditions (such as listed above) can last many decades.
This is why a full investigation of all such claims is required by the Church before people claim miracles. Many miracles have been later debunked as due to natural causes or, in some cases, diabolical intervention.
I wouldn't have thought of the antibiotics and medication in general, but that very well could have some sort of effect. (It might vary with the metabolism of every individual.) Also, there are all the preservatives and other additives that are in food nowadays. I have heard that bodies are not decaying the way they used to. Heaven only knows what all this stuff is doing to us. You need look no further than drastically decreasing testosterone levels in men.
These are all things that should be looked at, in determining whether incorruptibility is truly miraculous, or whether there could be some sort of natural reason, even if an unusual one.