I heard that I could study during the weekends (something "unthinkable" in Spain unfortunately).
I am interested on IT area.
Yes, but you would have to pay for it.
I have been working as a helpdesk operator in a callcenter for almost 5 years. My mother tongue is Spanish and I consider that I have a very good english level.
Is there really possibilities to find a job? I mean, with Spanish, English, French (basic) and Catalan?
Yes, you would be able to get a job in a call center.
I read some other posts where people say that 4000pln monthly is not enough to live decently (I share flat here, so I won't mind doing that in Poland).
About 4000 PLN monthly brutto is about the best you can expect, unless you have some rare, highly technical expertise. Your French and Catalan will not be too salable, but your Spanish will be, and your English will allow you to function in a call-center environment.
4000 PLN brutto is about 2600 netto. Probably not enough to live comfortably in Warsaw and pay for school.
You are unlikely to earn more unless you are a pretty advanced programmer or have a masters in engineering.
I would strongly advise against studying at any of the private schools. Study only at the University of Warsaw or the Warsaw University of Technology (Politechnika Warszawska). Generally, degrees from private schools are worthless even inside Poland, and worth even less outside of Poland.
Also, evening and weekend studies are held in lower regard both inside and outside of Poland, as are courses taught in English.
You would be much better off studying full time at a good school in Spain or the West, and picking a program that is more likely to land you a better paying job like petroleum, geological or biomedical engineering, econometrics, actuarial sciences, financial mathematics or financial engineering. Take a year off to brush up on your math before you start. Talk to your family about financing your studies.