Thanks guys for the help, really appreciate it - the problem with the initial setup is that the 'redists' will only get installed if you make a redists folder on DVD1 and NOT DVD4 - as REV0 explained.
This leaves the problem of there then not being enough room for the 'Redists' folder to fit on DVD1 as the redists folder on it's own is just over 400MB in size and the total size of DVD1 with the redists folder included is nearly 4.9GB which means it's over the size limit of a regular DVD
Any ideas PLEASE?