Oanda mt4 question

Beginner Questions
1

Hi,

I opened a demo account on Oanda and using it downloaded the platform MT4.
I installed the exe and it installed MT4 and Meta editor.

I would like to write my own EA’s etc.

All the help links in the platform refer to MQL5.

When I try to create a EA in the Meta editor and save it, It saves the file with the .mql4 extension.

My question is what language does the MT4 platform support. Is it MQL5 or MQL4. Which version should I learn.

Thanks.