Hamburg vs Bremen vs Cologne vs Dusseldorf

Here's some good advice: You can't go wrong choosing any one of those places. Whichever one you choose, you will enjoy, so don't sweat the decision.

To some extent it would depend on the day of week, e.g. obviously the bars in the Düsseldorf oldtown are busiest at weekends and much less brisk on weekdays (maybe unless during a major trade fair), you might want to consider this.

'Unique' is a rather vague label, IMO it hardly fits to any of those places you mentioned at all. Either specify more of your interests or do a search on this board, as all of these cities have been topics frequently.

Just to make it more difficult for you, I'll add Münster to your list, a friendly and bustling university town, summer term starts in April, so it might be slightly quieter there right now.

Bremen prob, ranks as nr 4 in the list, entschuldige wenn da Bremer Stadtmusikanten dabei sind, between the others and as you clearly dont know anything: toss a coin, you wont mis the others either, becse you dont know. there is not really THAT much to qualify between them on the points you mention, but they DO differ an enormous lo0t in the normal daytime things you can do there.......... and HANNOVER is the only big city really on the train line from ber to AMS

Thanks for the help. I guess something I should mention is that I am looking for a fairly compact city, that is easy to navigate and get a feel for in a day. Anyway thanks so much for your help so far.

None of these 4 cities is on the direct route. On the direct route Osnabrück would be an option. Also a nice hostel there.
Another option would be the Autostadt of Volkswagen in Wolfsburg.

Both have enough to offer for the planned time span. No must to choose only cities from the list of Germany's 10 largest cities.

i' d go for Hamburg! it's a great city, different to Berlin. and the nightlife there is excellent! you don't wanna miss out on the most famous red light district and party zone in germany, do you!? ;) when you go out there, go to Hans-Albers-Platz. there are heaps of little clubs and bars with free entry!

There is not a lot in Dusseldord, it is a "working"city with a very high foreign population and apart from the Altstadt, there is not a lot here.