Yeah - 4GB of RAM is pretty slim these days, and I'm not surprised that the OS is complaining. Fire up Activity Monitor (in the Utilities folder) to see what is actually going on.
Your MBP can take 16GB of RAM (Apple said 8GB "officially" because that's all that was available at the time - it takes 16GB just fine. We only recently gave away our mid-2012 MBP - and it's still running fine. We put an SSD in it too...) So you really need to decide how much you want to spend on it or whether it's time for an upgrade, whether new or used. Generally when I get to the end of the line with the OS, I move on, but then there are the software issues. I have moved on to Affinity Publisher entirely. Still use Photoshop and Lightroom though (but got Capture One and own Affinity Photo as well...) through my son's school account, but that's about to end and then I'll have to decide. Probably will start paying through the nose to Adobe, at least for a while; I just don't have time to learn Capture One right now.