If single frame in 4K takes about 4 hours to render, it's only natural to assume that the same frame in full HD will take 1 hour, since it has 4 times less pixels. Upgrading RAM will not help in the slightest if your system is not actually running out of RAM. Corona low memory warning is just that - a warning, it does not necessary means that Corona needs to use swap. If that would be the case, then your render probably would take 40 hours and more and you would see 100% disk usage and pretty low CPU usage.
As Tom already mentioned, your CPU is not very fast and 4 hours for 4K interior doesn't sound unreasonable. That being said, it's quite possible that your scene can be tweaked further to improve render times. Check the performance stats tab in the VFB, see if you have UHD/4K cache too low, or if rays/sample is excessively high. Maybe your background isn't set right, perhaps window glass blocks too much light, or maybe your albedo is too high. Maybe your materials could be simplified to save some time. Also consider rendering shorter and cleaning remaining noise with denoiser. In summary, do some scene inspection and try to find ways to improve render time - when rendering animation, every saved second counts.