Hi Hatchgeek,
The "4 x 100" is a bolt pattern of four lugs in a square pattern with a 100 mm diagonal distance. The wheel size for the base Fit steelies is "14 x 5.5" meaning 14" diameter and 5.5" wide (with a +45 mm offset), while the Fit Sport alloys are 15 x 6 (with a +55 mm offset). Check out this website from another thread:
http://www.rims-n-tires.com/rt_specs.jsp
On TireRack's website they feature many 14" and 15" wheels that will substitute for the originals.
For tires, the steel wheels come with "175/65 x R14 81S" tires, meaning 175 mm thick with a 65% aspect ratio of height/width, radial tires to fit 14" wheels, with further "81 S" tire characteristics-- check any tire manufacturer's website for defintions. The Honda alloy wheels come with 195/55 x R15 84H tires which are thinner but wider, than for the steelies, and can handle higher speeds & hotter temperatures. Again, many tires can replace the originals-- check with TireRack or your local tire dealer.
Hope this helps,
Bill Lakatos