Well, I understand why you went this route to achieve apparent 4:3 image, but technically it’s actually not correct. This is because the analog image from consoles is not exactly 4:3, and the 4:3 is achieved by cropping overscan by the frame of the screen itself.
Consider consoles outputting 256 or 320 dot active widths (eg SNES and MD/Genesis, respectively). They use 8:7 and 32:35 pixel aspect ratios. In both cases the horizontal width, when converted to square pixels (as we need to on a fixed pixel LCD display), is roughly 293:
256*8/7 = 292.57
320*32/35 = 292.57
This equates to the full line being multiples of 390 H samples (as is already the case for Lx3 and Lx4 at default settings):
341*8/7 = 389.7
427*32/35 = 390.4
Now if we want this to cover the entire horizontal space of 4:3 area, by necessity the image height needs to be
292.57/4*3 = 219.4 lines
So the ”most correct” 4:3 image is achieved by zooming on your TV so that the visible image is cropped to a multiple of 219-220 lines. If your display allows it, it could also be achieved by setting V.Active to those values on the OSSC.