In July it was revealed that Costco was selling a slightly better version of the Surface Go with 4GB of RAM and 128 GB of storage to its members. This device sat between the generally available 4GB/64GB and 8GB/256GB RAM and Storage version everyone else could get.
It seems that exclusive period is over and that model is now available to all customers via the Microsoft Store in USA.
Microsoft is selling the device for $499, but of course without a type cover, which will set you back another $99, making it slightly more expensive than the Costco version, which sold for $549 with bundled Type Cover.
While I am sure most users would prefer more RAM over increased (expandable) storage, the 128 GB storage is of the SSD kind rather than eMMC, which should bring its own performance benefits.
See the device at Microsoft here.