Please note: MT4 backtesting is garbage compared to MT5 backtesting. Also, it will be much cheaper and faster to backtest using MQL5 cloud network than to build a special-purpose PC for that.
As for your question, having a one-core powerful CPU is the best option for MT4 optimization. You don't need multiple cores as MT4 cannot multi-thread. You don't need much RAM for optimization (unless you have some weird EA that requires a lot of memory for its calculations).