If the AcO is a prodrug, for it to last so much longer at a similar intensity must mean that, compared to dosing psilocin directly, lots more 4-ho-DMT is working in the brain but spread out over time, kind of like time released psilocin. Let's assume both that the AcO is a prodrug and that some psilocin is destroyed before it gets to the brain, but that 4-AcO does not get destroyed before the brain because it's a more durable molecule. Also, assume that the dosage weights for the ho and AcO are, as you suggest, closer than I suggest for similar intensity of effects (say, the AcO is 20 percent heavier for equal effect- which is about the difference between the versions in g/mol).
So, assuming all else is is equal - deacetylizing enzymes are quickly active in the brain, etc. - for a similar intensity of effect 10 mg psilocin and 12 mg 4-AcO used IV or IM should feel about the same if no psilocin is destroyed in the blood since about 2 mg of the AcO is cleaved off in the brain. However, because we're assuming psilocin is destroyed in the blood it suggests that the psilocin dose should be >10 to be equivalent to 12 mg of AcO used IM to make up for the loss during breakdown in the blood. If for the sake of illustration we say 1 mg of psilocin from the 10 mg psilocin dose is destroyed in the blood that means 9 mg is available to the brain. If the AcO survives intact in the blood that means 12 mg of AcO is available to the brain, but since around 2 mg of the weight is removed on conversion to psilocin about 10 mg of psilocin is ultimately available to the brain. So if the end result of our dosing these amounts is 9 mg in the brain for the original 10 mg psilocin dose and about 10 mg of psilocin in the brain for the original 12 mg AcO dose (very close), then why should the AcO dose last so much longer while maintaining a similar intensity level? On these assumptions doesn't it make more sense to think the AcO is active on its own in the brain but it's not as potent and has a longer duration?
Despite this, I'm inclined to believe 4-AcO is a prodrug or at least that it doesn't reach receptors as 4-AcO, simply because, as you've stated, there is good pharmacological reason from the example of heroin to believe the standard explanation. I'm still not seeing how it adds up, though.