I don't think they would allow a fourth skater from a nation. Just a feeling here...
I think you're right. The majority of the member federations are the smaller ones who have just one skater at Worlds. Every additional skater from the powerhouse nations means the smaller ones' best skater has less of a chance of placing high enough to earn two spots.
The smaller federations aren't going to vote in favor of such a proposal, and there are many more of them than the powerhouse feds.