The honest answer is: for most traders, the platform is chosen by the broker, not the other way around. Still, it helps to know what actually differs between the three once you're automating a strategy rather than trading manually.
MetaTrader 4
The oldest and most widely supported of the three - if a broker only offers one platform, it's often MT4. It has no netting account mode, so every position an EA opens gets its own independent ticket, which makes multi-position logic straightforward to reason about. Its indicator system is simpler (no indicator "handles" to manage), which also means slightly less flexibility for advanced custom indicators.
MetaTrader 5
MT5 is MetaQuotes' newer platform: more built-in timeframes and indicators, and - importantly for automation - both hedging and netting account types. On a netting account, multiple same-symbol orders merge into one position at the platform level regardless of how your EA labels them internally. That's a genuine platform constraint worth knowing before you build a strategy that opens the same symbol from more than one rule.
cTrader
Popular with ECN-style brokers, cTrader's automation (cBots) is built on C# and an event-driven model - your code reacts to events like a new bar opening or a position closing, rather than checking conditions on every tick in a single loop. Functionally it can express the same strategies as MT4/MT5, just structured differently under the hood.
So how do you actually decide?
In order of what actually matters in practice:
- What your broker supports. This eliminates the choice for most people before the other factors matter at all.
- Netting vs. hedging, if you plan to run multiple rules on the same symbol at once - MT5 and cTrader ("Netted" mode) can merge same-symbol positions; MT4 never does.
- Whatever you're already comfortable navigating - if you already know one platform's interface for monitoring live trades, that familiarity has real value.
The good news is you don't have to commit to one before you've even tested an idea - a strategy built once here exports to MT5, MT4, and cTrader from the same set of blocks, so you can try it on whichever platform your broker actually offers.