I've never used business class, only 1st class and I've never had a compartment on a RailJet.

I recommend you lookup the number of the train you propose to get on https://www.vagonweb.cz/razeni/ and look at the details of the business/first vehicle. That site normally has photographs of most vehicles and depending on the train, it may take you to this page https://www.vagonweb.cz/fotogalerie/...mpz-8090-7.php.

There is also a newer RailJet but most observers think it's worse and I vaguely remember business class being specifically mentioned.

Under normal circumstances, reserving seats that will be unused doesn't work as (I think) reservations are cancelled perhaps 20 minutes after leaving the starting station so if there's nobody in the seat then anyone can take it. A certain amount of social engineering is possible to discourage people, but in the end, if 1st class would be really busy then all seats will be occupied. On the other hand, if 1st class would be quiet then you will likely get a bay of four to yourselves, especially if you get on the train where it starts.