Hacker Logic & Cipher Puzzle.
On a computer? Scan with your phone's camera to get the app.
Race another operator to the same code, or play a standoff where each of you sets the code the other has to break. Outsmart your rival and claim the reward.
Assemble mining rigs, expand your vault, and run overclocks. Your network keeps earning credits while you are away.
Join a syndicate, claim territory on the map, and take your share of the syndicate treasury that every raid feeds.
Break into a rival operator's grid and take what they left unguarded — then set your own defenses before they come for you.
Use advanced tools like Logic Analyzers, Time Warps, and Oracle Hints to gain the upper hand.
18 long-run contracts, fresh daily missions, and fifteen ranks to climb on the global leaderboard on the way to Server King.
A closer look at the app.
Version 3.0 turns Protocol 4 from a code-breaking game into a network you run: a new standoff duel mode, a mining empire that earns while you sleep, and syndicates that raid each other.
Two secrets instead of one — each of you sets the code the other has to break. Turns alternate, and a solve only ends the match once both sides have had the same number of turns, so going first never wins it. Solve on the same turn and the duel is level.
Assemble rigs, expand the vault, run overclocks. Your network keeps mining while you sleep.
Join a syndicate, claim territory, raid rival operators. A share of every raid now feeds the syndicate treasury instead of conjuring credits out of nothing.
18 contracts, daily missions, and a rebuilt rank ladder where competitive play finally counts. Reinstalling no longer wipes your progress, and there is a real iPad layout instead of a stretched phone.
Yes. Guess 4 was renamed Protocol 4 with version 2.0 in April 2026, by the same developer; the classic code-breaking game is still at its heart, now with PvP duels. Games released later under the name Guess4 are by other developers.
Yes. Protocol 4 is free to download on the App Store. Optional in-app purchases include credit packs, starter kits and one that removes ads.
A code-breaking logic game in the same family as bulls and cows. You guess a secret passcode and every guess returns readings that tell you how close you are, so each code is solved by deduction rather than luck.
There are two kinds. In a Race, both players crack the same passcode and the first one through wins. In a Standoff, each player sets the code the other has to break, and a solve only ends the match once both sides have had the same number of turns.
iPhone and iPad with iOS 17.0 or later. The game is available in English, Turkish, Spanish, French, German and Portuguese.
Not yet. Protocol 4 is currently available on iPhone and iPad only.
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