The three that did not burn through signify problems completing the spell due to past issues (social or emotional more than magical, usually) that were not fully addressed or that linger from previous situations.
The fourth candle finally burning through shows that you overcame the problem -- and the fact that it resulted in the glass breaking shows that you may have indeed broken the resistance to your work. It is a very definite sign. A broken glass is NOT usually "godod," but in this case it shows a break-through on the conditions of the past that hung around for three candles. Now it remains to be seen if the next candle burns through to the end and does not break. That would be an advantageous finale to a run of Reversing viigil lights.
See many more comments about reading glass candles here in the Lucky Mojo Forum:
Candle Divination: Flame, Wick, Wax, Glass, Remains
candle-divination-flame-wick-wax-glass- ... t8537.html
Learn more about signs from candles here, in my free online book, "Hoodoo in Theory and Practice:
Candle Magic Divination:
http://luckymojo.com/candlemagicdivination.html
Learn more about the art of burning magical candles in this practical book:
BOO-GRI-AHCM
The Art of Hoodoo Candle Magic by catherine yronwode and Mikhail Strabo
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You can order right here in the Forum by clicking on the blue Add To Cart button.
Good luck.