Almost two weeks ago, Microsoft released Windows 10 for PCs build 17074.1002 to the Slow ring. Typically, this would mean that the company would be publishing new ISOs soon, but unfortunately, Microsoft's Brandon LeBlanc confirmed today that that won't be the case. Apparently, there's a bug in the clean install process that makes it not possible.
That means that if you want to download ISOs for a clean installation of Redstone 4, you'll have to use build 17025, which is almost three months old. In fact, that's the only Redstone 4 set of ISOs that was ever released. The download page offers the last two, and the one previous is build 16299, the RTM build for the Fall Creators Update.
For now, everyone is going to have to rely on Windows Update to get the builds that they want. You'll have to do it the old-fashioned way, which is to set your PC to the Slow or Fast ring by going to Settings -> Update & security -> Windows Insider Program.
It's unclear when Microsoft will publish any new ISOs, as it has published Fast ring builds before, but it was two and a half months between the latest two Slow ring builds. In other words, don't hold your breath.