My question is, while I don't speak a word of Polish, can I get a job in Warsaw for 4 months?
Pretty much zero chance of finding work teaching for that time period, or, really, at all in your case.
The only possible job you could get is in a call center as "customer service". Not a pleasant job, and the pay is very low, but it's better than nothing. Actually, the wisest choice financially speaking is that you stay at home in Portugal for the four months and earn money there. It would be safest to make your plans based on the assumption that you will DEFINITELY not earn a penny in Poland. If that makes you panic, then don't go. Go only if you are fine with the idea of a four-month-long unpaid vacation.