Interesting, i actually find that i myself don't have hardly any nausea at all from 4-ACO-DMT, unlike mushrooms or oral DMT, although i haven't had nausea from oral DMT using Moclobemide, but i mainly go the Huasca route so Aya/Psilo-huasca, and so with the Huasca you have the Harmala-related nausea/vomiting so that's probably that, but i've never had nearly as much gut issues with Psilohuasca using 4-ACO than i have with Aya using oral DMT, i've even taken 4-ACO on it's own quite a few times and also never had nausea.
But, if you do have nausea, try getting ahold of some pure Limonene (you can find it on amazon, for example), and take like 10 drops of that in a capsule, either with the 4-ACO or like 30 minutes to an hour beforehand, ime Limonene is the active compound in Lemon essential oil that helps with nausea/vomiting, and my feeling is that it's because it helps to get digestion going, and also by being a Serotonin 1A agonist may inhibit something called Nk1 which apparently inhibiting that receptor can act as an anti-emetic stopping nausea/vomiting. Though Limonene also agonizes the Adenosine A2A receptor so you may try drinking a bit of Caffeine to antagonize the Adenosine A2A receptor, which can counteract the possible drowsiness Limonene may cause. But ime, 10 drops of Limonene seems sufficient enough to counteract the nausea/vomiting from Harmalas at least, so i'm sure it'll help with 4-ACO, maybe even mushrooms and cacti, idk. If 10 drops don't do it for ya, try 15 to 20, but 10 should be good, anything less than that doesn't seem to have the desired effect for me personally.
Oh, and the Limonene can also help with anxiety as well. Another good thing to look into, is 3 to 4.5 grams of dried Lemon Balm leaf tea, also helps with anxiety by smoothing out and calming down the come up.