Here a lot of the guys ride Bianchi's, and im friend with LBS who import them, so having a chance to compare some of them since i ride C60 my self.
C60 is far from sucking ur energy and beeing not stiff enough. It very stiff where it needed, but in same time very comfortable, so u have a feeling that is not stiff enough (since dont beat u on the way).
Sometimes stifness feeling is how u feel the road. if u feel every bump, every road buzz u got idea that is stiff. But feeling that not mean that u are actually faster.
Actually that is one of the best sides of C60. Beeing stiff and take all watts u can put, but comfy in same time. And when u compare ur effort later (same effort, same ride on different bikes), u realize that u are actually faster on C60 (i had that kind of comparation for example with Pinarello Dogma F8, and Pinarello Dogma 60.1 and Oltre XR2 (I had XR2 on test ride then for 10 days, and rode 500 km on that bike). Same wheels and tires on all of them.
I guess similar will be with XR4. Just feeling stiffer, but not beeing faster in reality.But this is assumption, which i can test ride for u if u want, since few riding friends have XR4, and can compare with my C60.
Ok, this is One part of the story
The other things u should consider:
C60 and XR4 are exactly the same weight (i have pictures of 50S C60 and Oltre XR4 55 frames. If u are interested i can post them, since i measure them before installation). So no weight advantage here
C60 is much higher quality then Oltre XR4 (and same weight). - For example on XR4 u press bearings directly to carbon, on C60 there is alloy threaded inserts and for me is best PF implemenation ever. Same story in headset part. C60 have alloy inserts, on XR4 u press bearings directly in carbon.
Reliability - Dont know how it is with XR4, but there was a lot of Warranty claims for XR2 and some other Bianchi's. I know 5 guys im riding with them, who had warranty issue on XR2. For XR4 still early, but seem like much better quality then XR2. C60 in this part will be much better bike.
Dont know in u'r part of the world, but here Bianchi have very good warranty.
Few friends who had crack in the frame, got new frame as replacement without any problems. The only problem was that one of them had to wait 3 months until he got his new frame (in his case he got cracked XR2, Bianchi did not have on stock anymore, so they offer him XR4 as replacament. More then fair enough
Size Choice - One of the reason i did not bought Bianchi (i wanted to buy Speciallisima in the time i was buying), and for me it's much better bike then Oltre (because at least is light, and i prefer the look of it) is the size choice which is very limited. In my case 53 cm was too small, 55 too big. Nothing in between. For C60 u can buy ur ideal size, which will lead to ur comfort and handling much better then some marketing hype about special material.
Oltre XR4 look on first sight very nice.. Very sleek. And maybe best looking aero bike in this moment. But since every bike look similar it's not distinctive. Also it's kind of design which look very nice now, but probably in year or two u'll be bored of it. C60 will look still nice in 10 years from now.
At the end.. If u still decide to go for Oltre XR4 (and check Specialissima just in case..Still thinking is better bike), go for Celeste Gloss. Much better looking then Mate (one friend have Matte colour, the other one Gloss)