Yeah, and if you actually pay attention to how he is, its pretty obvious he will take the opportunity

"it wouldn't be a surprise if you paid attention.

9:24 - "How'd you like to appear in a reality dating show?""

• he never said yes
• even if he did (he didn't), he can still change his mind, like he already did it multiple times about being or not an actor
• something else could have happened that make the story change direction

so, yes, I paid attention, but at no moment in the episode 4 it explicitly said "the next arc is going to be a reality TV show".
you could "guess" it, it was even the most possible thing to happen, but it was certanly not a 100% sure thing, especially considering how this is a show with possible plot twists.

but there can't be plot twists if some dumb people just spoil that this is going to happen.

I don't want to know that something that has 99% chance of happening is really going to happen, I want to discover it by myself.
is it too hard to understand ?

I'll give you a simple example with One Piece.
Luffy wants to be the king of pirates by finding the "one piece".
At the end of the series, pretty much everyone is guessing that Luffy is going to find the one piece.
But for now, we don't know, because the show isn't finished yet, maybe we'll get an unexpected ending, maybe not.
But if some guy that read the manga for example tells to the guy that watches the anime 6 months in advance that in fact he did found the one piece, what do you think their reactions would be ?
Sure, some people like you wouldn't care, but then the others who are 6 months late are now spoiled for the rest of the whole series, and considering that the show is like 20+ years long, this is kind of a big deal.

now, in the case of oshi no ko, it may not be a "huge spoiler" or a "huge surprise", but it still spoils me for probably the next whole month.
and considering that I already find the show pretty unsurprising so far, it only helps me hating this show (and its fans) even more.