New OS for Mac

Hello,

I have Mac Air with Software OS X 10.9.5 (13F1911) and 4 GB 1600 MHz DDR3 memory, which OS could the best option to run my PC smoothly and which of these are available for free downloading.

Thank you very in advance for your assist.

You should be able to upgrade to Sierra. In fact, it may be your only option. But with only 4GB RAM it may run slowly. Although Apple's specs state 2 GB RAM minimum, in 6-8 GB is a more realistic minimum.

Check the other specs:

MacOS Sierra - Technical Specifications

Check the other specs:

MacOS Sierra - Technical Specifications

Which is the model name of your MacBook Air?

https://support.apple.com/en-us/HT201862

Hi,

Mac Air 13inch, Mid 2013 Model , Processor 1,3 GHz Intel Core i5 and Memory 4 GB 1600 MHz DDR3.

Thank a lot to both of you, whose replied already.

If you are running OS X Mavericks, I don't think that the macOS Sierra will be noticeably slower...

I would go for it...

Get Ready for macOS High Sierra - Official Apple Support