When Microsoft first announced the Surface Go last July, there were only two models available. The higher-end one had 8GB RAM and a 128GB SSD for $549, while the base model included 4GB RAM and slower 64GB eMMC storage for $399. A couple of weeks later, it was found that Costco was selling an exclusive variant with 4GB RAM and a 128GB SSD.
Many regarded that third model as the sweet spot for Microsoft's budget tablet, as it was lower in price but didn't force buyers to sacrifice storage performance. 90 days after the Surface Go was released, so was Costco's exclusive on that model, and Microsoft started selling it.
As of today though, the configuration with 4GB RAM and a 128GB SSD haas disappeared from Microsoft's online store. If you head over there now, you'll only find the base model, the higher-end model with 8GB RAM and a 128GB SSD, and the LTE model that was released in November.
The good news is that if you want that model, you can still get it at the original place it was sold, at Costco. It still comes bundled with a black Type Cover for $549, and you can find it here. If you want to check out the Microsoft Store listing, you can find that here.