There are many more MT4 Brokers than MT5.
Some things are very similar, for example Object handling, File handling.
Indicators work a little differently . . .
You need to determine what you want from your coding, if you can do it in mql4 then do so . . . if there are things you want or need to do and they can't be done in mql4 ask and someone will probably know if it can be done in mql5 and advise you.
That's exactly what I wanted to know
I think it really depends on what happens when the new MT4 (currently in beta) is released. I have seen the new mql4 documentation it is heavily biased towards mql5 methods and features with little to no support for the currrent C based mql4.
If it remains that way it would mean the only support for C based mql4 will be this forum and whatever old documentation is still available. I think I would recommend start learning the basics of mql5 because the new mql4 will be based on the same thing with legacy support for C based mql4 functions and over the long term it will be beneficial to know some mql5.
I have a little coding experience and MQL4 looks simpler to me.
The simpler to code, the easier to focus on trading strategies.
A professional programmer surely will think different,
but I really don't need OOP (object oriented programming) in a 1400 lines script.
The best language would be the easier to describe a trading strategy.
But this is subjective, relative to your programming skills.
Experts love details to increase their control over the process.
I also think mql4 is much simpler to use, easier to learn and well suited to the task. I have not seen an explanation of why MQ felt it neccessary to implement an OOP language for this, but I think as that is the way things are going it would be beneficial to get up with it. I have only just begun to dabble with mql5 so I really could not comment on the benefits of using it because I have yet to learn what they are. Having said that the little I have learned already has served to increase my interest in it.