This essentially sounds like a way of transferring production from other cities to the wonder-producing city. Building a 'builder' will cost x production. If that builder can perform 4 'builds', then a simple transfer of production from one city to another would allow the builder to expend x/4 production with each 'build' on a wonder tile. I reckon this would be powerful enough even if there was some loss of production in that transfer, so it might be the case that each 'build' progresses the wonder by x/5 production, or something.