Yes, it's the potting soil. I'd do heavy water changes if it were me. I few back to back to get that ammonia down to 0, then daily large ones. A planted soil tank cycles faster than normal but it still needs it's time and if he wants the fish to live then the effort must be done until the tank cycles.
The Tetra safestart normally calls for no water changes but I don't trust it, especially not in a 4ppm ammonia.