I think you need more detailed information from your potential employer about the structure of your salary (I'm sure they know the Polish laws well enough to provide you such detailed info). Because if you have to pay both pension and health insurance from this money the remaining 3500 PLN won't be much for 2 persons. What about the other person? Will she/he have the medical insurance as well, you can't take risks and pay full hospital costs for her/him in case of emergency.
Normally Polish laws allow to divide the income with your wife (or your child that is minor, under 18 years old). So you would be paying a little less tax, but if the person isn't your family, I'm not sure how it works. And I don't know if this rule applies to foreigners in all cases.
andy b:
Using the above calculator, a gross (brutto) salary of 5000 PLN per month means a net (netto) salary after tax of 3500 PLN per month.
maybe, just maybe, it would be the salary after National Insurance?, then the tax would be about 800 PLN.