Well, if my memory is correct, the auto-system should go into "cruise-mode" within 0.25psi of the selected cruise altitude (4000' in this case)
If you would transition before this point to 1013.25mba, this might indeed trigger the "off scheduled descent" warning as FL04 is lower than 4000', thus you never reach this 0.25psi difference.
On the other hand, I might be completely mistaken and just made a fool out of myself
will check my manual as this is an interesting one.