I think i understand the question allough i may go off half cocked here.
On the left of the CDU is the LNAV profile, so long as u are on a good intercept heading LNAV will engauge and the aircraft will follow the profile.
On the right is the VNAV profile, 4000A indicates 4000ft at or before the waypoint, the FMS will calculate the required climb/decent rate to attain this requirment, climbimg the FMS will recalculate through the Thrust Management Computer the best thrust required from the engines to get you there. On decent the FMS will prbably command idle hold and may be more drag wich will appear in the scratchpad of the CDU. If the calculations dont add up the the scratchpad will show unable next alt. Meanwhile LNAV will continue with its profile the 2 modes are totally independant.
Now this if off the top of my head i could be inacurate in some areas but i think thats the gist of it.
Stu.