Ok, fresh install of OMV only or fresh install of OMV + Linux (=format my server) ?
The version of Debian Linux that each OMV version is running is different (except for 1.x and 2.x). So, a fresh install requires a fresh install of the OS (use the OMV 4.x iso). While this will require formatting the OS drive, it will not format the data disks (they shouldn't be connected when installing).