B733- Off schedule descent
 

Fairly mundane problem in this case.
Off schedule descent light came on (and master caution), despite cruising at the prescribed altitude for 10minutes.
B737-300, Cruise at 4000' for a short positioning flight. Cruise level set at 4000'. QNH was 1034.

Am I correct in thinking this is because of the large difference QNH to std?

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.

It has only happened in this aircraft, and then only the once. We were at 4000' for 10minutes, pressurisation was set at 4000. When we got descent to 3000', after passing 100' below, the warning came up.