Unfortunately, there really just hasn't been much scientific research done on these kinds of things, so no one can tell you for sure because we don't know ourselves. However, if I had to make an educated guess, I would bet that it has something to do with dopamine.
If you're wondering why anything makes you feel good, dopamine is probably the safe bet. Even MDMA's rewarding properties appear to depend on dopamine based on animal studies, just downstream of serotonin release. In other words, even though the effect is dependent on serotonin and thus sensitive to depletion, it's still probably not the serotonin itself that makes you feel good technically, but the dopamine released in turn by that serotonin. And there's probably also a slight synergy with MDMA's weaker dopaminergic stimulant effects as well.
The thing about psychedelics is that they feel like MDMA because they bind directly to serotonin receptors, making them serotonin mimics. However, because they bind to the receptors with different affinity and efficacy ratios compared to serotonin itself, the overall net effect on neurotransmitters will be still related but ultimately different. For that reason you could end up with a psychedelic that feels similar to MDMA but less euphoric, or similar but more euphoric, or even more euphoric but dissimilar for other reasons, and so on. 4-AcO-MET (probably actually the 4-HO-MET it metabolizes to) would probably then just be one of the more euphoric forms.
And as for psychedelics not causing depletion, it's probably because dopamine is significantly harder to deplete than serotonin, likely because it's far more crucial to biological survival, being a key factor in learning and motivation. Take, for instance, the fact that one good roll can leave you drained, whereas people generally abuse stimulants for much longer periods of time before it truly starts to get to them. By comparison, the intense but temporary and short-lived dopamine release by psychedelics would be almost nothing.
Those are my thoughts on the matter anyway.