How to convert an MT4 EA to MT5

With the .mq4 source code, the EA can be rewritten for MT5. With only a compiled .ex4 file, it can't be converted: there's no readable code inside to work from. In that case your options are the original author, or rebuilding the EA from its rules.

1. 1

   Orders become positions, orders and deals

   MT4 has one pool of orders. MT5 separates pending orders, open positions and executed deals, and accounts can be netting (one position per symbol) or hedging. Every piece of trade-management code has to be rewritten, usually around MT5's CTrade class.
2. 2

   Price arrays disappear

   MQL4's ready-made arrays like Close[], High[] and Bars don't exist in MQL5. Prices are read with functions such as iClose() or CopyRates().
3. 3

   Indicators work differently

   In MQL4 you call iMA() and get a value. In MQL5, iMA() returns a handle you create once, then read with CopyBuffer(). Every indicator call in the EA changes shape.
4. 4

   Error handling and order checks

   Return codes, filling modes and trade-request checks differ, so logic that 'just worked' in MT4 has to be re-tested in MT5's Strategy Tester.

1. 1

   Rewrite it yourself from the .mq4 source

   Realistic if you have the source and know MQL. Plan for a rewrite, not a find-and-replace, and re-test everything. MQL4 vs MQL5 explained.
2. 2

   Hire an MQL5 programmer

   The usual route for complex EAs. Give them the .mq4 source and a written description of what it should do. What that typically involves.
3. 3

   Rebuild it from the strategy rules

   If you know what the EA does (its entries, exits and risk rules), you don't need the old code at all. Describe the rules in StratoBot and it builds a fresh MT5 EA. How that works.