I think a C-Mount or PL mount adapter will open up your choices for wider lenses. The C-mount is definitely going to be a cheaper adapter and options for cheaper lenses. The PL is going to be a little more substantial in cost and quality.
At 4.5mm, I wonder whether any lens that size will cover the frame. So, you might get the same end-result from a 7 or 8mm that covers the full frame and doesn't require digital cropping to remove the vignette. Of course if you intentionally want that fish-eye effect with the severe vignetting (ie for skateboard movies) then that's another story.
Basically, if you want to avoid having to digitally crop, then you want to look for lenses that will cover the Super-16mm frame. For C-mount TV style lenses, the key to look for is that you want one made for a 1" sensor (not 3/4" or 1/2" for example). Unfortunately not all sellers list or even know this information. The only problem with these lenses is that they are very small (like sometimes the diameter of a quarter). So they are not practical for pulling focus. But, at that wide often everything is in focus anyway and it might even fit in your pocket!
Here's just a few ideas from both sides of the spectrum currently on eBay:
http://www.ebay.com/itm/SOLIGOR-TV-LENS ... 20d08497afhttp://www.ebay.com/itm/Optar-illumina- ... 0955278964