I can't imagine *any* tryptamine or phenethylamine is banned. Those are two of the most common classes of neurotransmitter and a huge variety of OTC and prescription drugs fall into one camp or the other. Melatonin, for example, is N-acetyl-5-methoxy-tryptamine.
The US has what's called the Analog Act, and 4-aco probably falls under it, but it's not a very well-written law so it's almost never enforced on users, only dealers, and even then it's not always worth the time and effort to make the charges stick.
Since 4-aco metabolizes into psilocin, using it is still unambiguously illegal. I've read posts on other forums claiming that 4-aco will oxidize in small amounts into psilocin on its own in normal conditions, in which case they would have a much easier time pressing charges.
Drug law is ridiculous in this country. It's almost impressive in a bizarro world kind of way for a law to simultaneously be so draconian and so vague as the Analog Act section of the Controlled Substances Act. Luckily for recreational users, possession charges that aren't linked to intent to sell or distribute are almost always handled by state law, and many states don't have anything like the Analog Act. Federal laws are much tougher, but you pretty much have to be a high profile dealer or get really unlucky to make yourself into a federal case. That's my understanding, anyway; I'm not a lawyer and (knock on wood) I've never had a possession charge of any kind myself, so I may be misinterpreting things.