After unlocking Tier 4, you have a good amount of story to go, you must complete the main story before unlocking the final Venture - the Saints Tower. This will cost you $8,000,000. You may have enough by the time you complete the story if you've been completing all the ventures and doing other stuff so you have accumulated enough income to afford it. If not, complete all the ventures first then work on other trophies until you can transfer enough cash. Once you have enough you can return to the Venture table and purchase the tower. Talking to the building manager once you're at the tower will start the 'Best Friends Forever' mission/cutscene and will earn you both this trophy and We could go home, or… .