btw, using a dithering to create a '5th' color is exactly how you would go about it without going beyong 4 colors. it's still 4 colors isn't it. It's just perceived as more. Well done.

That said, there appears to be some sort of anti-aliasing in your image which makes it have way more then 4 colors.
Also, the transparency gradient of the dithering (the sky dots) makes it way more than 4 as well.

this is how it should look I think in 4 color ....

did steemit do something to compression ?