please help choosing city to study (Magdeburg, Bielefeld, Ulm, Essen)

There is no one place better than the other - they all have their individual strong and weak points. Therefore it is impossible to give you a clear recommendation here. Best you visit the places yourself to make up your mind from first-hand impressions!
Student jobs are generally easier to find in the South of Western Germany, still o.k. in the rest of Western Germany - and worst in the East. Unfortunately, the quality of life (for a foreigner) and also the cost are higher in exactly those places, where you are likelier to find a job. (With Konstanz being one of the most expensive cities in Germany, due to its proximity to Switzerland and Lake Constance - a rich man's playground!)
Before you make any plans, please do check whether you can work at all on your visa, whether your course of study allows you the time for that (full-time classes generally do not) and whether you have the required (i.e. good) German language and other skills to find anything but the worst paid menial jobs (like supermarket shelf stocker or harvest helper).
To get the student visa, you also need to show sufficient financial reserves (currently EUR8700/year). If you are thrifty, this should be enough to survive, so you might not need to work and can concentrate on your studies (which is advisable!).

I know Tübingen is expensive and Darmstadt less so. But one should really look more at the program and what a city has to offer rather than just the prices! The thing is that price levels are just an average and may not be reflective of what you will actually find. A city can have a lot of expensive property which raises the statistical price level but the places students re going to live in are not so expensive.

One can inquire about official student housing which is usually fairly cheap but in limited supply in most University towns. Most students end up getting rooms in a private shared apartment, (known as a Wohngemeinschaft or WG) usually but not exclusively with other students. This is usually much cheaper than finding one’s own apartment but means getting along with other people, which can obviously have advantages and disadvantages. Best to find people with similar lifestyles. If one  is quiet and a non-smoker, non-drinker then they probably won’t get along well living with a bunch of loud partyers.

All Universities are going to have pin boards where people hang up ads looking for rooms or for roommates. One should always inquire if a price includes utilities (warm). Also, one might pay more in an apartment but beyond getting a room there might be a common living room, TV, internet access, washing machine etc. included. So the question is not just the price but what you get for your money.

Thanks for your responses, i was actually in the strong and weak points of each option, as beppi was saying. But of course, i'm mostly interested in comparing the positives.

Regarding the programs, after what i researched, I find all of the Unis in those cities strong. I forgot to include Bamberg as well. The comparison between Tuebingen and Darmstadt is precisely my point and why I would choose Darmstadt. I don't think during the study period I will need distractions, but mostly a quiet and comfortable place to live, where I can enjoy some hobbies I have, particularly music and going to libraries and walking around the city.

I appreciate any info and pros and cons you may have, cheers

What else do you want to know that wasn't already mentioned above?
In the end you need to do your research and make a decision yourself.