One of the key difference between mt4 and mt5 is the programming language. In mt5 the language used to develop EAs and Indicators is MQL5.
Though MQL5 and MQL4 may sound similar, but MQL5 is completely a different language than MQL4 and of course, mql5 is NOT an upgrade of mql4 which some may think.
Usage of mql5 has many big advantages which are almost impossible to achieve in mql4 as well as few disadvantages at the same time.
The main disadvantage is that mql5 is different from mql4 and there is no easy way to convert your favourite EAs, indicators to mql5 when you switch from mt4 to mt5. So you need to program from scratch or hire a programmer.
The main advantage of Mql5 is that it uses object oriented programming and the syntax is very similar to c++ and so if you already know c++, then you can quickly learn mql5.
There are few very important strategies like advanced machine learning or Artificial intelligence which can be easily implemented in MQL5 which is almost impossible to achieve in mql4.
Artificial General Intelligence