Where are Streets of Rage 4 sale, discounts?

Streets of Rage 4 is on sale of course.

Maybe you'll get lucky and it will eventually join the sale. Until then ... well ... Fight'N Rage is on sale if you need a substitute.

It's last sale, according to ITAD, was back in April.
And since the game doesn't have much consistency with its sales here, it becomes hard to even try to predict when will be the next time it goes on sale.
My suggestion is to put the game in your wishlist (heart symbol below the price and "add to cart" button on the game's store page) and wait for it to go on sale. GOG should notify you when that happens.
Meanwhile, maybe you'd want to look for beat 'em up alternatives? (heads up for Mother Russia Bleeds, btw, since it was delisted from Steam out of the blue recently, meaning it could get removed from here soon as well)

I've been playing since the Atari age and now I care about graphics. I know that indie games do a great job of artistic work with pixel art and I don't want to belittle that work but these days I prefer to play games in high definition. I bought shing here on gog and found it to be a fantastic game.

Best thing you can do is add Streets of Rage 4 to your wishlist and make sure you have the e-mail notifications enabled, so you can get alerted when the game is available on sale.

I would also recommend Fight'N Rage if you're looking for something similar. It plays in high resolutions with different filter effects for preference based visual style.

And yea, 'Shing' was a pretty cool game. I have that here also and enjoyed my time with it.

It's one of those games that doesn't go on sale that often, so it can be easy to miss if you don't check on GOG regularly. However, since the summer sale going now, hopefully SOR4 will be available for a discount.