I believe that gear 4 is a 1:1 ratio on your 722.4 transmission. With a 1:1 ratio and a 2.87 rear end, you should be doing about 100 mph at 4000 rpms. Also, the 722.4 does not have a lockup torque converter, so there may be an extra 100-200 rpms of slip in the torque converter.
All this being equal, I would think that you would be over 4000 rpms at 100 mph and probably be running close to 3000 rpms at 70 mph.
Hopefully, someone else with personal experince on the 722.4 transmission in the C280 can step in and verify if this is the case.
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