Forza Horizon 4 discounted

Forza Horizon 4 and if you use gamepass trial you get a nice discount

All depends if you want to own and keep Forza Horizon 4. There is an advantage to getting Game Pass, in that not only will you get access to Forza Horizon 4 but also over 100 other games to play, whilst you are subscribed. You also get a discount on the
games, if you wish to purchase them whilst subscribed to Game Pass.

Usually forza games go on sale when the developer makes a big annoucement let's say there will be a high chance that the game goes on sale after E3

Xbox sales change every week, keep an eye out and it will be on sale soon I'm sure.

Games go on and off sale all the time, frequently with no advance notice. Sales start normally on Tuesdays at or around noon UTC and go until the following Monday. Major sales are once a quarter or so (except for Black Friday and Countdown, which are in
November and December, respectively), and the next one should be the Spring Sale, which hopefully will begin in the next week or so.

To come back to the Game Pass statement: yes, Forza Horizon 4 is on Game Pass now, but sometimes players decide to discontinue access to Game Pass. Knowing that, you can still purchase the game, even if it's on Game Pass. What makes this even better is a 20%
discount on all games that are currently on Game Pass, so if someone had GP and wanted to buy the game, it's 20% off regardless of a current sale.

(Of course, if a sale is running, the 20% discount most likely won't apply; that is, you can't stack discounts.)