Yeah it seems to vary a lot depending on the TV. Maybe it’s using PC mode in 4x which seems softer on certain TV’s. I’m getting a picture on my Samsung using 5x with 320×240. Have you tried both 1920×1200 and 1600×1200 to see if one works better than the other? The H Samplerate would have to be about 427-428 to eliminate those problems but as you said the TV isn’t accepting that.
4x on my samsung looks good when I use 320×240 mode because it scales a perfect 4x with black borders.
The Panasonic EX700 (4k) I have is very fussy but I can get NTSC working with generic 4:3 in 1366×768 and it does look sharp to me. I know Samsung TV’s have a habit of making PC mode very soft so maybe your Sony TV is doing that in 4x since 1280×960 is likely picked up as a PC resolution. You’ll know if that’s what causing it if 3x actually appears sharper since it’s 720p and more standard than 960p.