The blind spots
What MT4 cannot do
MT4 has three blind spots that matter the moment you move from one EA to several. These are not bugs — they are the natural consequence of the Strategy Tester being designed around the unit case. The questions below are the ones traders actually ask, with the actual answers.
Can MT4 backtest multiple EAs at once?
No. Strategy Tester is single-symbol, single-EA. There is no built-in mode where you point MT4 at three EA files and tell it "run all of these on one $10,000 account from 2020 to 2024." It does not exist in MT4 or MT5.
The workaround traders try is opening two Strategy Tester windows side by side and running EA-A in one, EA-B in the other, with a $10,000 starting balance in each. This produces two separate reports that look like they cover the same period, and it's tempting to add the profits together and call that the "portfolio result." It isn't. Each EA in this setup got a full $10,000 to itself — not the shared $10,000 the live account would have. There was no margin competition between them. When EA-A drew the account down 8%, EA-B's lot sizes did not shrink in response, because EA-B was on a different simulated account. The drawdowns never overlapped onto one balance, because there was no single balance to overlap on.
What you get from running two Strategy Testers is two solo reports. What you'd get on a real shared account is a different equity curve entirely — usually worse, sometimes meaningfully worse. The Strategy Tester has no mode that closes this gap.
What's "drawdown stacking" and why does MT4 miss it?
Drawdown stacking is what happens when two or more EAs enter losing periods at the same time on a shared balance. Their individual drawdowns no longer sit in their own private equity curves — they hit one account at once, and the account drops by the sum of the simultaneous losses, not the worst of them.
If EA-A's worst drawdown is 12% and EA-B's worst drawdown is 15%, the naive expectation is that the portfolio's worst drawdown is roughly 15% — the deeper of the two. That intuition is wrong almost every time. With moderate correlation between the two strategies (say, 0.4), the portfolio's worst drawdown will land closer to 22-28%. With higher correlation, it can be 1.5x to 2x worse than the deeper individual drawdown. The math behind this is covered in detail in EA Portfolio Drawdown Stacking.
MT4 has no way to model this because each backtest is independent. The Strategy Tester does not know that EA-A had a bad July at the same time EA-B had a bad July. It cannot, because each EA is running in its own isolated simulation. The temporal overlap of drawdowns — the single most important variable in portfolio risk — is invisible inside MT4.
What about correlation between EAs?
MT4's report shows individual EA stats but cannot compute portfolio-level correlation between two or more EAs' returns. There is no "select multiple reports and show me a correlation matrix" feature. The Strategy Tester is built to summarize one strategy's performance — it has no concept of "another strategy's monthly returns" to correlate against.
You could, in principle, export every trade from every EA, line them up by date in Excel, compute monthly returns per EA, and run =CORREL(...) across each pair. A few thousand rows and ten formulas in, you'd have a correlation matrix. Most traders never do this, partly because it's tedious, partly because it's easy to get wrong (return windowing, daily vs weekly, log returns vs simple returns), and partly because the problem isn't visible until live trading reveals it. The dominant assumption ends up being "running EA-A and EA-B together gives me roughly the average of their solo results." That assumption is what blows accounts up.
The cleaner question correlation answers is: when EA-A loses, what happens to EA-B? If the answer is "EA-B is also losing," they are not diversifying you — you have one strategy in two costumes. If the answer is "EA-B is roughly flat," you're getting real diversification. MT4 cannot produce that answer for you.
The summary in one line: MT4 is fluent in single-EA backtests and silent on everything that happens when those EAs share an account. Adding individual reports together is not a portfolio backtest. It is a mathematical shortcut that consistently understates real risk.