If 4th gear is 'drive', i.e. 1:1, and the rear end ratio is 3.46:1, then assuming a tire size of 195/65R15 - probably smaller than actual in this case - the engine speed at 60 mph would be 2800 rpm. At 70 mph it would be 3260 rpm.
Now, I don't know how accurately you noted your engine speed or road speed, but note that 6/7 X 4000 rpm = 3429 rpm for 60 mph, which is not what you indicate. I would recheck your figures, and if they are accurate, than either you have problems with your instruments, or your transmission is slipping.