FCSExplorer,
You owe some food to IThoughtTheredBeFood
BUT
If you set an MCP altitude higher than your your planned descent path and engage VNAV (Vnav descent now),you aircraft will go into an Energy Compensation ,reducing your airspeed .If the speed reaches a too low value,vnav will disengage as it wont be able to recapture your vnav profile.
You must set your Missed approach altitude when it is MINIMUM 300 feet ABOVE your current aircraft altitude to allow Vnav to continue its descent.
I believe you mentioned 4000 ft as being your Miss App alt, therefore you should set it not at 4000ft,nor at 3900 or 3800 but when you pass below 3700Feet....AND if you are within 2nm of the first approach waypoint(when your FMC transitions to "ON APPROACH" logic.
If you fly VNAV approaches I suggest your read Flight Navigation Systems, appropriate limitations, the FCTM and follow your Airline SOPs
In any case don't forget temp corrections next winter
