Not exactly.
What's the difference between a 4 lug wheel and a 5 lug wheel except that one has 4 holes and the other 5?
I've went to 5 lug exactly because my 5 lug 17' bbs rs are lighter than my present 4 lug 16' MAS wheels. It's all about the type of metal used.
For eg. i've also got a set of 15' OZ Fittipaldi wich are as light as the 17' bbs. 
The difference is that most 5 lug bmw rims are heavier than their 4 lug counter parts since the 5 lug BMW specific wheels have to be made more durable(hence more material) to handle the heavier weights of the BMW cars generally over the Japanese cars.
Also, you are not comparing apples to apples when comparing something like MAS or Fittipaldi to BBS. Try comparing SSR to BBS to be more fair.
Broaden your scope and do a search between 4 lug rims made for the Japanese cars vs. 5 lug BMW rims and you will see a difference. I'm not saying in all cases but there are a lot of the 4 lug ones that are generally lighter than 5 lug BMW specific rims.