Vier
Four in a row, for iPhone and Apple Watch.
You against the device — no account, no network, no ads, no analytics. Vier opens straight into a game and picks up where it left off, and the watch app runs on its own, without the phone nearby.
A real opponent
The computer plays a negamax search with alpha-beta pruning, at Easy, Medium and Hard. Every level takes a winning move when it sees one; the harder ones also block yours.
On your wrist, on its own
Move the disc with the Digital Crown and drop it with a swipe. The watch keeps its own game and its own settings, with no phone required.
Two players, one device
Switch to two players on the home screen and pass the phone back and forth, no opponent to beat.
Nothing to sign up for
No account, no network connection, no ads, no tracking. It remembers only the game in front of you.
Also on Linux
The desktop version, from the Excelano apt repository, so apt upgrade keeps it current:
curl -fsSL https://excelano.com/apt/setup.sh | sudo sh sudo apt install vier