Port Royale 3

This will most likely come naturally once you have progressed further into the game and have established multiple trade routes and businesses. 

TIP: Constantly check in with a town’s administrator or governor as they often provide tasks that require little work but reward large sums of gold. As you progress and rank up, you can be rewarded with 200,000+ gold per task. Another strategy that will net you large sums of gold is annexing towns through force. Each time you annex a town you can usually loot up to 500,000 gold.

NOTE: You need to have the 4 million on hand, as having an overall worth of 4 million won’t count.