Here is one experiment that attests to the safety of 4-ACO-DMT Fumarate. My friend did around 55mg total, different routes of administration, and he is okay and he said it was awesome.
First, he took a 25mg capsule orally.
He starts tripping, about 2 hours later decides that he wants another capsule.
He splits the second 25mg capsule in half. He eats half of it (~12mg) and snorts the other half (~12mg).
About 10 minutes after the second administration (remember he already has done 50mg), he decided to do an IV dose of about ~5mg. He just immediately broke into laughter/tears and got big rush from it, loved it.
So, based on seeing my friend do this, I have much more confidence as to its safety. This is in no way saying that it's safe, it's just a hypothesis of mine and the reason for forming it.