What use is this 4000' 'MSA' contour in a MSA sector of 10,000'?
Say you were coming in from an angle, or were cleared down by ATC, you could check your radial/distance and see whether it made sense, if there wasn't a published vectoring chart or to back it up if there was. Our quadrantal MSA (SSA) is determined by the highest obstacle in the "cheese", which may be up to 30 miles away and 90degs off course. Nice to have a map with this kind of thing on (which is the only good bit on the Navtech charts).
MSA for us = Minimum Safe Altitude, which gives approved vertical clearance within a specified area (envelope). Sector Safe Altitude (SSA) = quadrantal application of same.