Complex sentence: When the AmigaOne boards first became available OS 4 was not ready and they were supplied with various Linux distributions, from April 2004 boards were shipped with a "developer prerelease" version of OS 4.
Split into simpler sentences: When the AmigaOne boards first became available, OS 4 was not ready: they were supplied with various Linux distributions. From April 2004 onwards, boards were shipped with a "developer prerelease" version of OS 4.