worth the jump to Cornwall IV's from Forte IV's?

CW IV are good, no doubt. But LS are better. You might try to recalculate your available  real estate, lol.

Yes, you are right. The CW IV is full range and the LS is not. But that horn loaded bass driver when mated to a good pair of subs is a thing of beauty.

I agree that Cornwalls don’t plumb the depths but they have very defined bass down to the upper 30s. I used subs with mine, but they weren’t necessary to get very satisfying bass. And as @mwjones stated, the bass of the LS can be surprising too. I can certainly see why some people don’t use subs with them depending on your musical taste.

I know this isn’t exactly the same thing, but I had Forte IIIs, moved to khorns, then to Spatial Audio X3s to CW IV. I think the Cornwalls were significantly better than the FIII.