One of the big questions to ask is: which platform does my broker support?
Not all brokers that support MT4 support MT5.
So if your broker only supports MT4, it doesn't make sense to make your EA's using MT5, because they are not interchangeable.
There are some interesting and significant differences between both platforms.
I wrote an article about the differences between MT4 and MT5 and submitted to MQL for review. I havent heard from the moderators for about the publishing of the article for weeks. It would have helped.
Thank you. have a good day.