Identifying my Current 15 Inch Wheels - and thoughts about wheel replacements

No offense meant, but again, your terminology is wrong. You're talking about BACKSPACE, not offset.
4 1/2 inches of "offset" would put it against the leaf spring, or if the other way, sticking out the fender two inches.
A + or - offset of 25 (mm!) would be about one inch. 4 1/2 inches of offset would be around 110 mm.
See if you can find a wheel like that.