I am using PS CS4 on a 2012 Mac Pro using OSX 10.7.5. For some time I have used this with 16 GB of RAM in the Mac. Recently I started some work with 32 bit images and began getting 'out of memory' or couldn't complete an operation because of insufficient memory. So I purchased and installed and additional 8 GB of RAM for a new total of 24GB. Nevertheless the Performance page in PS stills shows 3072 MB of available RAM. Why would this not have increased on addition of the additional 8GB of RAM to the computer? Have I wasted my money on the additional RAM?
Photoshop CS4 Mac installs a 32-bit version only. Less than 4 GB available to CS4.
An upgrade is in order. Lightroom and Photoshop | Adobe Creative Cloud Photography plan
Since photoshop cs4 is only an 32 bit application, it's max memory use is limited to around 3 gb of ram.
If you want to take advantage of your added ram, you'll need an 64 bit version of photoshop cs5 or newer.