FVG indicator for MT4

There is no MT4 version yet. This page says what exists and what does not.

Planned An MQL4 version. No date, and no source file exists for it.

Why the MT5 file does not run on MT4

MT4 uses MQL4. MT5 uses MQL5. The two languages differ in how an indicator receives its price arrays and in the functions that draw objects, so the MQL5 file does not compile in MT4's editor. Do not try to rename it.

What you can use today

The browser detector on the home page. Export or copy candles from MT4 as open, high, low, close lines and paste them. It applies the same rule.
The MQL5 source as a reading guide. The rule is two comparisons, so a port is short. If you port it, keep the comparisons strict and read only closed bars.

The rule an MT4 version would follow

Bullish gap: the low of candle 3 is above the high of candle 1. Bearish gap: the high of candle 3 is below the low of candle 1. Wicks only, strict, and only once candle 3 has closed. The same inputs would apply: see the settings page.