Generally, less travelled routes are assigned 4 letter numbers and most important flights have one or two digit codes. All of NW's TATL and TPAC flights (international only) had two digit codes. I don't ,mind three digit codes as they are not hard to remember. I have a great deal of difficulty remembering 4 digit codes.
Note: By single digit, I mean one significant digit, such 001, 009, etc.
I would like to see some single digit, and two digit (0xx) type flight numbers. The most important routes could be single digit flights.

Lately, DL has changed flight numbers often that I don't remember any of Delta's international flight numbers. KLM has made very few changes on its long haul flight numbers over the last 35 or 40 years. Short, easily memorised flight numbers do have an appeal and encourages repeat business from not so frequent fliers.

Does anyone have any opinions on this?