For institutional-style FX execution at $1M+ sizes with minimal slippage or delay, look at brokers/ECNs with deep liquidity rather than typical retail shops. Firms like LMAX Exchange, Currenex/FXSpotStream, Integral, CFH Clearing/AMP Global, and Dukascopy JForex are known for professional liquidity and straight-through execution; many support MT4 bridges or can connect via FIX/MT API. Pure STP/ECN providers with robust LP pools tend to handle large flows better than standard MT4 brokers, so prioritize firms with true ECN pricing and multiple LPs. Confirm connectivity options, regulatory jurisdiction, and execution model before committing.