Understanding the Types of MQL4 Programs: A Guide for MetaTrader 4 Users

If you've ever stared at the MetaTrader 4 platform and wondered about the difference between all those files you can attach to a chart, you're not alone. New users often load up their platform, see options for "Expert Advisors," "Custom Indicators," and "Scripts," and assume they all do the same thing. They don't. And understanding the distinction isn't just technical trivia—it changes how you interact with the markets and manage your trading workflow.

MQL4, the programming language built into MetaTrader 4, allows for the creation of three distinct types of application programs: Expert Advisors (EAs), Custom Indicators, and Scripts . Each serves a specific purpose, runs differently within the terminal, and has unique limitations. By understanding these categories, you can better organize your trading tools and even appreciate why certain third-party utilities—like the Camovia Tray app—work the way they do to simplify your daily routine.

The Three Core Types of MQL4 Programs

Expert Advisors (EAs): The Automated Traders

An Expert Advisor is the most complex of the three program types. It is a program coded in MQL4 that the client terminal calls to execute on every price tick . The main purpose of an EA is to automate trading operations. Once attached to a chart, an EA can analyze market conditions and send trade orders directly to the trading server, effectively managing your positions without manual intervention .

The execution model of an EA is continuous. Once attached, it enters a tick-waiting mode; each new price tick triggers the program to run through its algorithm . This makes EAs suitable for strategies that require constant market monitoring and immediate reaction to price changes.

Custom Indicators: The Analytical Tools

Custom Indicators are programs designed for graphical display and market analysis. Unlike EAs, they cannot perform trading operations automatically . Their primary function is to process price data and display the results—lines, histograms, or arrows—on the chart.

Like EAs, custom indicators run on every tick . However, they differ in their initialization. When attached to a chart, a custom indicator executes its initialization function and then runs its start function once to perform the first calculation before switching to tick-waiting mode . In contrast, an EA waits for the first tick before executing its start function. This distinction ensures indicators are immediately visible on the chart upon loading.

Scripts: The One-Time Executors

A script is a program intended for a single execution of specific actions . Unlike EAs or indicators, a script does not wait for a tick to arrive—it is launched immediately upon being attached to a chart and runs completely through its code once, after which it is automatically unloaded .

Scripts are useful for one-time operations: opening or closing a batch of orders, deleting all objects from a chart, or placing multiple pending orders at once . Their single-run nature makes them lightweight and efficient for tasks that don't need ongoing monitoring.

File Types: How MQL4 Programs Are Stored

Understanding MQL4 programs also requires knowing the difference between the file types that store them. The platform works with three primary file extensions:

mq4 files contain the source code of a program. These are human-readable text files that can be edited in MetaEditor . However, you cannot run an mq4 file directly in the terminal.

ex4 files are the compiled, executable versions of MQL4 programs. When you compile source code in MetaEditor, it generates an ex4 file with the same name . This compiled file is what the client terminal actually runs. Importantly, the extension ex4 does not indicate whether the program is a script, EA, or indicator—it applies to all three types .

mqh files are include files—blocks of source code that can be reused across multiple programs through the #include directive . They help avoid duplicating code and are stored in the terminal_directory\MQL4\Include folder.

Where This Matters in Practice

Understanding these distinctions becomes practical when you're managing multiple tools on your MetaTrader 4 platform. For instance, if you frequently switch between different EAs or use several scripts for routine tasks, you'll want tools that simplify how you interact with the terminal itself.

This is where Camovia Tray comes in as a complement to your MQL4 setup. Rather than keeping the full MT4 window open constantly, Camovia Tray turns your terminal into a system tray tool. You can check live quotes by hovering over the tray icon, view open positions, and even close trades—all without keeping the MT4 main window visible . If you're someone who runs multiple EAs across different charts, or you regularly use scripts to manage orders, being able to hide the terminal from the taskbar and Alt+Tab view while still monitoring your positions can streamline your workflow.

Camovia Tray also supports the bridge for MT4 users: a one-time setup with the CamoviaBridge EA gives you access to position management from the tray . And because the app reads all data locally from your terminal, your quotes and position data stay on your machine—nothing gets uploaded . For traders who rely heavily on MQL4 programs but want to keep their workspace clean and less conspicuous, this adds a layer of convenience without interfering with the trading logic of your EAs or indicators.

In short, MQL4 gives you powerful tools—EAs for automation, indicators for analysis, and scripts for one-off tasks. Camovia Tray helps you manage the environment in which those tools run, making the experience more discreet and accessible.