Zombies, Run! celebrated the release of version 4.0 earlier this week with some major modifications. The popular gamified exercise app, which used to retail for a flat $4 at the Google Play Store, has switched over to a free-to-play subscription model. The app and gameplay themselves haven't changed much. Users are challenged to run (literally) a series of 200-plus missions while avoiding the gnashing teeth of an Undead swarm, all within the context of a post-apocalyptic audio drama. Six to Start, the app's developer, has already released four "seasons" of added content since its debut and will likely continue to do so. However, users will now have to pay $3 per month (or $20 per year) to keep getting these regular updates.