To clarify, further to our quick tutorial on installing mq4 files in MetaTrader – if you’ve downloaded an expert advisor or forex indicator or similar that is in the form of a ex4 file, it’s even easier.
All you have to do here is find the right folder and copy the file to it.

For example, it’s most likely your Metatrader is at C:\Program Files\MetaTrader, so all you need do here is drop it into

C:\Program Files\MetaTrader\experts

if it’s a downloaded expert advisor, or

C:\Program Files\MetaTrader\experts\indicators

if it’s an indicator and it should all show up next time you run MetaTrader.

And that’s it. What you can’t easily do with an ex4 file is look at its source code and edit it to your own requirements – there used to be one or two reverse-engineering applications on the market that would decompile an ex4 to an mq4 – they were, and probably still are, (incredibly) expensive for what they did.