Same here.
I had 18GB free on my C: drive before installation, but when I initially tried to download UE4.5 I kept getting the message “insufficient disk space” intermittently, first to say I needed 11GB free, then 10GB free, then 8GB free.
It may be coincidence, but every time the message popped up it seemed to say that it ‘requires’ whatever diskspace I happened to have free.
e.g. the very last popup said installation required 8GB, and that was the space I had free on my C: drive.
Nonetheless, the download progress never actually stopped, so I kept clicking [OK] to the messages. Eventually it installed regardless.
Perhaps the messages are a false positive of sorts.