Čtyřka is mariáš – the Czech trick-taking card game – for four players, in the browser against three computer opponents. The game is usually played two against two, but the partnership is hidden: the forehand picks trumps by naming a card, so only one other player knows whose side they are on, and the rest have to work it out from the play. Around that sits the bidding that can multiply the stake before a card is led. The game is free to play and installs as an app. It deals every contract the rules allow, keeps the money, speaks the calls aloud, and tells you why a card is not legal – or lets you break the rule on purpose, if you ask it to. It is a rewrite of the Windows shareware of the same name, released in 1999. The card play is that game's improved AI.