It will be better if you could share your birth details so that I can suggest you the right gemstone based on your problem.
Regarding the life span of any gemstone; yes, the gemstone will stop giving out its positive energies when it has fully lost its capability of fighting with negative energies. It doesn’t need to be 4 years, even it may loose its entire life with in few days. It mainly depends upon the quality and size of the gemstone (similar to your phones battery).
Regarding the color of Blue Sapphire; even sometimes you may get light blue shaded AAA quality gemstone. So its always better to get a gemstome with certification.