NW-S4

The NW-S4 was the first model in Sony's sports-oriented S Series. It used 64 MB of built-in flash storage for ATRAC3 playback, with a one-line monochrome OLED display and up to 40 hours of use from a single AAA battery. It introduced the S Series as an active-use branch within the early flash Walkman lineup.

The change was mostly about use case. Instead of treating every flash Walkman as the same compact digital player, Sony gave this one a clearer role around movement, durability, and long battery life. It was simpler than later S Series models, but it established the idea that a flash Walkman could be shaped around how and where people carried it.

That made the S4 the starting point for one of Sony's more flexible Walkman branches. The later S line would become richer and more varied, but this first version set the active-use logic in place.