First of all, no, by something resembling my theory they'd be 160\4 after 40 overs. However, I never actually said "each 10 overs of the innings have 40 runs scored and 1 wicket lost.
There are different places at which to aim if you want to bowl economically at various stages of the innings. For most of the first 40 overs, that's the top of off stump. In the last 10, it's the Yorker length, varying in line depending on if you see the batsman giving himself room.
Even if batsmen don't give their wickets away, there are lines and lengths, especially with the wicketkeeper standing-up, that it's simply not possible to score that quickly from, with a decent field.
If you can't contain batsmen in the last 10 overs, it's not just because they're "set", it's also because your bowling is not good enough.
OK, it's not the easiest task aiming in the blockhole ball after ball, but it's certainly possible. West Indies have been doing it surprisingly well in the present series.