How fix fonts on external tv look good view? i have a little problem with my mac and external tv (LG 4K). 

Macbook air 2012 (intel HD 4000)
LG 43UJ6307

i connect macbook with MiniDisplayPort -> DVI -> HDMI (1.4). by default i cannot set resolution to 2560x1440 (max resolut. is fullHD) but i fix it by switchResX app (but this only app can change this resolution more than fHD). when i set resolution to 2k fonts are vert uggly and readble. when i connect my Desktop pc and resolution to same size fonts are pretty and more understandable. (in 2k and 4k). 

i was disable in Settings -> General -> Use LCD Smoothing when available 

How can i fix it ?

i saw thereis option for HIDPI but there is only two variants 
960x540 (HIDPI)
850x450 (HIDPI)
It sounds like your TV is not correctly connected to the Mac, so that you only have Full HD resolution available instead of the UHD resolution you want.

I would suggest trying with a different cable from mini-DisplyPort directly to HDMI. This should give you the least amount of problems connecting to the TV. Ensure that the cable specifically supports 4k.

I have seen multiple times that even cables sold as HDMI 1.4 cables do not really support 4k with the Mac (I guess some companies deliberately re-label old cables). Specifically I have seen this also with expensive cables, where as a cheap HDMI cable from IKEA works without any problems.