jimecade
What is the latest or best OS version for my MAC air 4 gig RAM Intel core I5 processor early 2014, 128 SS HD?
MacBook Air 11″, macOS 12.1
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Per Mactracker the latest you can use is 11.6.2 Big Sur. That's the most modern available to you; it has all the latest security fixes for that version. It is therefore also the most secure version you can use.