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

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 think the radiators on your speakers give you good low end response relative to the CW IVs. I haven’t heard the Fortes but I owned the CW IVs And they’re not bass machines. They will play loud and clear and with dynamics. They will not shake the walls unless you are really loud. They don’t go as low as less sensitive speakers. They just don’t. My current speakers have three 8” (92 dB) subs and they are noticeably better at reproducing bass (in the same system). 

Check your measurements again. The La Scala about the same width as a CW and  is only 10" deeper. It can also sit flush against the wall.

And as @ozzy62 pointed out, the sound is something to behold, subs or not (as in my case). My advice is to not sneak up on what you want; just plan and go for it. It will cost less in the end.

Keep the Fortes. An upgrade would be Volti. 

I have replaced Cornwalls with Audio Kinesis "Jazz Module" (you can see that speaker on YouTube in my system composed of Atma Sphere amplification and TEAC vrds CD player). That particular model is not in production right now but shall be available again in october. Few other models from AudioKinesis will do a better job than Cornwalls (namely Azel and Gina with subs) particularly in the rendition of the stage / space. More importantly, you can contact Duke (the guy behind Audio Kinesis) and discuss with him about your needs. A better amplifier may be something to look into, as sometimes good speakers like Forte's are underexploited by poor amplification. La Scalla and Volti are very fine, but too big for any room and, let's be honest, ugly.