I am going to teach MQL4 language next February on 2017. But I read MQL4 no more use, it will change to MQL5. So how do I decide this training? If I teach next february, then after few months mql4 language no more useful?
I guess you mean "learn" (and not "teach").
If so, I recommend you learn mql5 directly. Almost everything you learn (except for commercial functions and some event handler functions that do not exist in mql4) will work for mql4 (while it is still in use), but not vice versa.
Learning mql5 will serve both the present and the future. Instead mql4 will only be worth it for you for the moment. If you start in February, and taking into account that getting a good level will usually take about a year, when you have gotten it, maybe mql4 will not be used too much.
In my opinion, the documentation on this site:
They did create the language, therefore, they are who better knows how it works.
You can download it in pdf format, too:
Regards and Merry Christmas.