Sounds like the Dutch zipcodes are very well organized, nice. The Danish 4-digit postcodes only define the city.
We've had centralized sorting for many years, though, with just four regional centers handling the physical mail - so even if you send a letter from/to Skagen (the northernmost part of Denmark), it takes a roundtrip through Aarhus. For packages, there's only two regional centers.
95% or more of all letters (I'm not of today's stats) are sorted at these four centers; the bulk is handled automatically by Siemen's Incentive OCR software and some Really Frigging Big MachinesTM. The stuff Incentive is unsure about is handled by humans, but still done on computers, with a pretty nifty system. And the rest, the really nasty incomprehensive stuff, is "city-sorted" manually, hands-on.
The end result is that the city post offices/distribution centers get a bunch of trays with pre-sorted stacks of letters, and the individual postmen can just grab their stack(s) of letters, jump on their bikes, and know that the letters are sorted in route order. Of course there's always a few letters that have to be hand-sorted at the local post offices, but that's because there's so many MORONIC, FSCKTARDED, PARKINSONS-DISEASE-INFLICTED, HEADSHOT-WORTHY individuals who have incomprehensible writing, get postcodes/housenumbers/streetnames/names wrong, et cetera. "To grandma & grandad, the yellow house at wurnglyspeltstrnaym" - and missing the postcode because they don't feel it's necessary because they live in the same town. Please go shoot yourselves.