MQL4 vs. MQL5: The Developer's Guide to MetaTrader
The Logic of the Upgrade: Why the New Language?
In the world of MetaTrader development, the transition from MQL4 to MQL5 is not just a version update—it is a complete architectural shift. MQL4 was built as a simple, procedural language (comparable to C). MQL5 was built from the ground up to be a modern, Object-Oriented Programming (OOP) language (comparable to C++).
While MQL4 is still legendary for its simplicity and dominance in the retail market, MQL5 is the engine of the future. It offers significantly faster execution speeds, a multi-threaded strategy tester, and the ability to trade non-Forex assets like stocks and futures in a way that MT4 simply cannot handle.
Key Archetypes: Procedural vs. Object-Oriented
MQL4 is Procedural. You write a sequence of functions (OnTick, OnTimer) and manage everything step-by-step. It's easy for beginners to grasp because the logic is linear.
MQL5 is Object-Oriented. You create 'Classes' for your trading logic, indicators, and execution modules. This makes your code more modular, re-usable, and easier to scale. If you are building complex institutional-grade algorithms, MQL5's structure allows for a much cleaner development lifecycle where you can swap out components without breaking the entire EA.
Comparison Checklist
Language: Procedural (MQL4) vs OOP (MQL5)
Execution: MT4 is single-threaded / MT5 is multi-threaded
Speed: MT5 is significantly faster for data processing
Market State: MT4 is 24/5 / MT5 supports stocks & futures
Infrastructure: MT5 allows for 'Cloud Network' optimization
Orders: MT4 uses 'Orders' / MT5 uses 'Positions' & 'Deals'
The Strategy Tester: MT5's Secret Weapon
The biggest practical difference for any bot developer is the Strategy Tester. MT4 is single-threaded; if you want to test 10 pairs, you must do them one by one. In MT5, you can test multiple currency pairs simultaneously, and you can 'Walk Forward' the optimization automatically.
Furthermore, MT5 supports Cloud Computing. You can 'Rent' thousands of CPUs from the MQL5 Cloud Network to run massive optimizations in minutes that would have taken weeks on a standard MT4 laptop. This makes the R&D process much faster for anyone serious about algorithmic trading.
Compatibility: Can MQL4 Run on MT5?
No. The two languages are not compatible. You cannot use an .ex4 file on MetaTrader 5, and vice versa. While the syntax is similar, the functions for entering trades (OrderSend vs CTrade) are completely different.
If you are a new developer, learn MQL5 first. It is the modern standard, and the coding principles you learn (OOP) will be directly transferable to languages like Python or C++, whereas MQL4's procedural style is becoming increasingly obsolete in the professional quantitative world.