I have Om4,
Can I charge to osmo mobile 4 with my iphone 11 adapter (18w) ?
What  the maximum  wattage adapter must use ?

yes you can

Hello there YukselErdogan. Thank you for reaching out and for this inquiry. You can charge DJI OM 4 via the Type-C USB charging port on the right side of the handle. The recommended charging current is 2 A. In addition, the DJI OM 4 has a charging time of 2.5 hrs. ( Measured with a 10W charger ). Thank you.

Indeed you can