MetaTrader 4 was released in 2005 and remains in wide use. MetaTrader 5 followed in 2010 and has become the platform on which new Expert Advisors are built. They share a look, a company, and a broker ecosystem, and they run different languages, MQL4 and MQL5, so an EA compiled for one cannot be attached to the other.
The differences that matter
MT5 has twenty-one timeframes to MT4's nine, which lets a strategy use two-hour, six-hour or twelve-hour bars. Its strategy tester can test several instruments at once, which is essential for a multi-market system, and it models ticks more realistically. MT5 offers both netting and hedging account types; MT4 is hedging only. MT5 has a built-in economic calendar, depth of market where the broker supplies it, and a more capable programming language. MetaQuotes stopped licensing MT4 to new brokers in 2022, so brokers that appeared since then offer MT5 alone, and existing MT4 brokers can continue but not expand.
MT4's advantages are habit and inventory. Many traders know it, and thousands of old EAs exist only in MQL4. Neither matters for a new system.
Why new EAs are MT5 only
A developer writing today writes in MQL5 because it can do things MQL4 cannot, because the multi-instrument tester is the only honest way to test a portfolio, and because the platform's future is MT5. Maintaining two versions doubles the work and the bugs. An EA that is offered for MT4 only is either old or built on a strategy that never needed MT5's features, which usually means a single pair on a single timeframe.
A buyer with a long-standing MT4 account finds a multi-strategy EA that trades eight indices, ten pairs and gold, on timeframes from one hour to weekly. It is MT5 only. The buyer's broker offers MT5 accounts. The choice is to open one, at the same broker, and move the trading there; the MT4 account can stay open for anything still running on it. There is no way to run the EA on MT4, and no reason to want to.
Check the platform before buying, and if the EA needs MT5, open an MT5 account rather than looking for an MT4 version that will not exist. Where the broker offers both, MT5's account types, tester and timeframes make it the better home for any automated strategy. An investor password, a read-only login for verification services, works the same way on both.