where did you get those wheel weights from :S
15" oz ultra at it's lightest offset weighs a shade under 5kg's, you'll not find a single road tyre in 15" that weighs 4kg's, most 15" tyres would weigh around 7-10kgs
so your looking at 12kg's minimum and thats being optomistic.
Reallistically most 15" wheel/tyre conbinations weigh as least 14kg each
even the light weight 13" race wheels and slicks i used to run on the single seater weighed more than 9kg (10.5kg to be exact)
I agree weight can be saved from careful choice of wheels and tyres, but you'll be looking at approx 10kgs max in like for like size change between all 4 wheels/tyres.