How to use Sacrificial Heart after finishing Act 3?

Basically the title, finished act 3 and realized I still have the heart which needs to be sacrificed at the altar in Aggorat, which I can't get to anymore after reaching act 4. I have found answers for people in the same situation in 0.2, but those solutions don't seem to work anymore as the portal stops working after starting act 4. Any help would be greatly appreciated, I can't even throw it in my stash so it's stuck bloating up my inventory.