MT4 EA

What an MT4 EA is

MetaTrader 4, called MT4, is a trading platform that many retail brokers offer. It lets a program, called an expert advisor or EA, run on a chart and place trades. An MT4 EA is simply an EA written and compiled for that platform. It follows the same loop as any other: read the prices, test the rules, check the guards, send or change an order, then wait. The forex EA page describes that loop in detail.

The word MT4 matters because platforms do not share EAs. A program written for MT5 will not open on MT4, and the other way round. The two use similar but different languages, and the platforms handle orders and testing differently. If you are shown an EA, the first question is which platform it is for.

MT4 against MT5 in short

• Different files. MT4 uses .mq4 and .ex4. MT5 uses .mq5 and .ex5.
• Different order handling. MT4 works with individual orders. MT5 can net positions or hedge them, depending on the account type. Details vary by broker.
• Different testers. MT5 has a tester that can use real tick data and several cores. MT4 has an older tester. The full comparison is on MT4 vs MT5.
• Different availability. Which one your broker offers is the broker's choice. Some offer both.

The files an MT4 EA uses

When you look inside the MT4 data folder you find a few kinds of files. Knowing them tells you what you have been given.

• The .mq4 file. The source, as readable text. Anyone with it can read the rules and change them. It sits in the Experts folder of the data folder.
• The .ex4 file. The compiled program. It is what the platform runs. It cannot be read back into rules, so an EA given as only an .ex4 is a closed box. You can test it, but not see why it trades.
• The .set file. A saved list of the settings, called inputs. It is the lot size, the stop, the hours and so on. Two people with the same EA and different .set files run different bots.
• Include and library files. Shared code the EA calls. If one is missing the EA may refuse to load. The Experts tab of the platform log says so.

To load an EA, the files go in the right folders and the platform must be allowed to run automated trading. The steps and the checks are in install an EA.

Where MT4 testing falls short

The MT4 strategy tester replays past prices through your EA. It is useful, but it does not copy live trading. Four limits matter most for an EA.

• Price data inside a bar is built, not recorded. MT4 history is stored as bars. The tester fills in the ticks between the open and close by a fixed method. The report shows a modelling quality figure that says how much of the test used better data. A low figure means the path inside bars was guessed. An EA that trades inside bars, with tight stops, is the most affected.
• The spread is fixed. The tester usually charges one spread for the whole run, taken from a setting or the spread at the time of the test. Real spreads change over the day, as the figure below shows.
• No slippage unless you add it. The tester fills at the price it shows. Live orders can fill at a worse price. See slippage.
• History quality is up to you. History downloaded from a broker may have gaps. A test over a gap is a test over a made-up line.

Worked example: one EA, four assumed spreads

This is a made-up EA, not a real one. It earns 1.8 pips a trade before spread, over 300 trades. The only thing that changes in the table is the fixed spread the test assumed.

At 1.0 pip the EA looks fine at 240 pips. At 2.0 pips it is behind by 60. Nothing about the rules changed. Calculated by us on 29 Oct 2026, exact. Now look at the figure. If the real spread is 1.0 pip most of the day but 3.0 at news time and 6.0 at rollover, a test with one number gets the quiet hours right and the busy hours wrong. For an EA that trades at news, that is the part that counts. The spread page covers this in more detail.

How to test an MT4 EA sensibly

• Use the best history you can get, and read the modelling quality figure in the report. Treat a low figure as a warning.
• Run the test at more than one spread, including one wider than usual. If the result flips, the edge is thin.
• Add commission if the account charges it. The tester does not always include it unless you set it.
• Keep part of the data for a second test with the settings fixed. See forward testing.
• Then run it on a demo account, and compare the live trades with the test. The backtesting page gives the whole sequence.

What this page does not tell you

It does not say MT4 is better or worse than MT5, or that any MT4 EA makes money. The EA and all numbers are made up. How your broker handles orders, spreads and account types varies, so check its terms.