MT4 vs MT5 for automated trading

The incompatibility is total

MQL4 and MQL5 are different enough that no conversion tool produces a working result for anything non-trivial. An .ex4 file will never appear in the MT5 Navigator, and this catches people who assume the platforms are versions of each other rather than separate products — see EA not in Navigator.

Practically this means the choice of platform is the choice of EA library. You are not picking a chart package, you are picking which programs can run.

The backtester difference matters more than the feature list

MT4's strategy tester is single-threaded and its modelling is well documented as producing optimistic results relative to reality. MT5 supports real-tick modelling and multi-threaded optimisation, which does not make a backtest trustworthy but removes one specific source of inflation.

Neither tester models spread widening, slippage or the broker minimum lot realistically without deliberate configuration, so the assumptions that inflate results apply to both.

The account model catches people migrating

MT4 is always hedging — multiple positions on the same symbol coexist independently. MT5 may be netting depending on the broker, in which case opposing orders offset into a single net position.

An EA written assuming hedging can behave unpredictably on a netting account, which is worth checking before running any unfamiliar system. See hedging.