Q: I have a MacBook running OS 10.9 with 4GB of memory free.  Can I install Yosemite?

What you need for Mavericks, OS X 10.9 & Yosemite 10.10

Other things being equal, 4GB is enough to check your email and run Yosemite. If your Mac runs well with Mavericks it should run well with Yosemite.

You probably won't be happy with the performance of your Mac with Yosemite if you are planning on doing other things that are more demanding on your hardware, like Photoshop or editing video, or using virtualization software. Unless you like seeing the spinning beachball.

Other factors are important too, like your HD (SSD vs. conventional drive), how full your HD is, how fast your processor is, and if you have some types of "crapware" on your Mac, for example, CleanMyMac or MacKeeper, or other similar dubious "utilities".  Or antivirus software. But that is a whole discussion in itself.

You can easily test drive Yosemite in one of two ways:

1. Make a backup of your current installation, like with Time Machine. Then if you don't like Yosemite you can restore from your backup.

2. Make a test partition on your hard drive and install Yosemite there.

U R welcome.