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I am running10.10.5 in my MB 4.1.
Apple would not approve more than Lion but thanks to MacPostFactor and A LOT of efforts I managed to reach Yosemite.
Of course there are less and less applications still working in Yosemite but much more than those possible with Lion. :)
 
Hi there. My 2015 early 13inch mbp still working Yosemite 10.10.5 perfectly, almost 6 hours battery life or more longer. But just run these apps, iwork office2016 iTunes safari chromium ibook mail onedrive f.lux Grapher, basically never hot even in summer day except watch video website. Apple provide battery replacement for 10 years, so if replace battery in 2026, it still works 5 years.
 
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