Now this may sound weired ... don't shy the why!
 
If I have say FC4 CD's, using this to boot, I arrive up to 'Installation method' prompt and lead it to iso images of FC5 (when available) on HDD partition FAT32,   will this work?
 
Alternatively
 
I have FC4 up and running, I get FC5 images (when available) and put them on HDD lets say anywhere accessible to FC4 (exclude NTFS), can a clean / fresh setup of FC5 be initiated from within FC4, on free space available on HDD (to give multi-boot). -no way - is it!
 
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Anil Kumar Shrama