You need to consider the quality of the scaler in your monitor. If you present the monitor with a signal that matches its native resolution then it will not need to do any scaling and you have full control over the picture quality. If you would like a softer vertical resolution then it might be good to take advantage of the monitor’s scaler since it might do some kind of bilinear resampling which the OSSC is not capable of doing.
So for 1080p displays, 4x is too much since it will generate more than 1080 lines. It is however possible to use 4x on displays with 1200 lines and then add a few black lines until you reach those 1200 lines. I have a 3x config for my Full HD TV and the above 4x config for the 1600×1200 monitor. I could use 3x on the Dell but then I’d have quite small picture with a lot of black around it since I want to have integer pixel mapping.