Imagine you have a milk bottle - or any bottle - and it's full of water. That water is the data all of your cameras are producing at any given time. When you pour the bottle out - no matter how hard you shake it - the neck of the bottle restricts the flow - and no matter what you do - the water will only pour out at a given rate.
Your NVR has this bottle neck - so that only a certain amount of data can be processed at any one time, hence your getting your 'no resource' screen. You have to reduce your image quality/frames per second etc. until you reach a point where all the data passing through the bottle neck is doing so unhindered. The best way I found to overcome it - was to slowly nudge down settings one at a time - quality/fps - until the problem went away.
I'm not sure on your particular model - but when it's unlocked - and you have the quad screen on, and you hover over the bottom of each of the camera screens, a number of icons usually appear - one should be 'switch to main stream' or 'switch to sub stream' when you hover over it - when you're on the quad screen -what are you set to? If you have all four as 'main stream' - try changing one or two to sub stream. On quad screen you don't usually spot much of a difference in the viewing quality (and the recording quality is unaffected). When you go full screen on any camera it's usually main stream automatically.