Although I know where you are coming from regarding some of your points about four, except the last bit ;-) I think its misleading.
The undertail would better suit a Kloss's type I think and I initially thought thats what it was, however, to me it appears structurally wrong. YBW undertails can also be deceiving and look similarly white -
http://orientalbirdimages.org/search.php?Bird_Image_ID=71648&Bird_ID=1837&Bird_Family_ID=&Location=
The light is harsh which doesn't help, but the bill is weak and dark tipped on the lower mandible, like a YBW. The flaring super and eyestripe also fit, as does a slight coronal if actually there. Your right on the lack of dark centred coverts or the what appears to be a lack of a dark base to the secondaries but I'd also give some bleaching factor there to both the light and wear. I'm not convinced the tertial edges would be obvious at this angle
I could be wrong and certainly erring on the side of caution, but for me. #4 doesn't look like a Blyth's.