although i do not have one yet and bearing in mind that cleaning a sensor is also not as easy as 1,2,3... i would rip it and take it to a shop where they clean stuff like that.
only disassemble it to the point where one just snaps it open.
of course only if you do not want to send it back to the wonder costumer service DJI where it will be close to never coming back before the end of this year lol...
so, either rip and have it cleaned at a shop or semd it back to have it cleaned or replaced...
cheers and just my 2c. no offense meant.
As the X4S has a mechanical shutter, can it be locked open to get a sensor swab and Eclipse fluid on it to clean it? That or a blower bulb maybe as the first attack method. I clean my DSLR sensors all the time and it's sort of scary the first time, but after a while not so bad. I also use a lighted sensor loupe to see the spots. For whatever reason, most all of my sensor debris are small white specks (Pollen?) and they result in a black spot on the image, usually in the sky.
The X5S I believe is open as it has no shutter so it might be easier to clean. Dirt gets on them at times no matter how much you try and keep it clean, especially if it has removable lenses. Rough part is video sensors get hot and some substances melt to the sensor so a wet clean is about the only way to get rid of it, hence the Eclipse fluid which dries quickly and is generally streak free over the alcohol/water-based ones which smear for me.
I haven't seen anything about DJI sending you back a refurbished or used camera if yours goes in. Wouldn't surprise me if they did swap it out though.