I thought it would be a good idea to have a central location for people to have a resource of information on the platform and programming similarities and differences between MQL and MT 4 and 5. I was going to originally start just a thread for discussion, for people to post things they have noticed, but I have seen that people have done that already in articles and topic threads, so I am including a link to many of them here, to make it easier for people to find. Feel free to add to the link list any I missed, and/or continue to offer helpful tips and recommendations here.
They are in no particular order.
https://www.mql5.com/en/articles/1201 Plotting trend lines based on fractals using MQL4 and MQL5
https://www.mql5.com/en/articles/117 How to Order a Trading Robot in MQL5 and MQL4
https://docs.mql4.com/mql4changes Updated MQL4 (from the MQL4 documentation)
https://www.mql5.com/en/articles/81 Migrating from MQL4 to MQL5
https://www.mql5.com/en/articles/66 Transferring Indicators from MQL4 to MQL5
https://www.mql5.com/en/articles/45 The Drawing Styles in MQL5
https://www.mql5.com/en/forum/120344 Migration from MQL4 to MQL5
https://www.mql5.com/en/articles/344 Using MetaTrader 5 as a Signal Provider for MetaTrader 4
https://www.mql5.com/en/articles/189 How to Copy Trading from MetaTrader 5 to MetaTrader 4
https://www.mql5.com/en/forum/142858 Why some great coders and trading system developers are ignoring Metatrader 5?
https://www.mql5.com/en/forum/146027 mt4 on mt5
Hopefully this thread is useful as a resource for budding programmers, and maybe even some more experienced ones as well.