That make sense. I personally would not use a full bed of 4g either. In a hybrid it is stiff, but the comfort level is a lot higher than say, cofocus. I know that sounds crazy but it really is true for me.
It sounds like the challenge will be softness versus durability. And that boils down to how often you want to restring your gut mains. 4g is one of the few polys that lasts the length of a hybrid setup for me (roughly 2 weeks). It is also not as soft as other strings that last half as long.
Anyway, let us know if you try black magic crosses. I think you may be really happy if it lasts long enough for you.