Problem is finding an older version of LR, IE LR 4 or 5. I don't think you would have a problem using LR 6 but it might be slightly slow. But you should be use to that already as 4GB of RAM is the bare minimum, and I do mean bare, for Yosemite.
thank you for your answer, I will consider getting more RAM before updating, Cheers!
A bigger issue is your vRAM (video RAM)
NVIDIA GeForce 320M 256 MB.
- 1 GB of Video RAM (VRAM). 2 GB of dedicated VRAM is suggested for large, high resolution monitors, such as 4K and 5K-resolution monitors.
Run the 30 day free trial version before committing any cash to the software on that machine.