I work 4 days on and 2 days off, and get paid 40 hours a week. This doesn't add up to 40 hours, so every 3 months we are forced to come in on our own time for 4 hours to make this time up. (there is a quartely average that we owe 4 hours, or 15 mins a day) This may sometimes make the work day 12 hours. Is it legal for them to do this? Should there be some kind of overtime?

Generally speaking, yes there should. What, in fact, the employer is doing is something similar to comp time, but in reverse. The private employer cannot offer comp time in lieu of overtime, so it stands to reason that they can't do the opposite either. The fact that they are paying you for 40 hours per week when they you don't work 40 hours per week (although, if you are working 4 10-hour days, I don't understand how that could happen) does not relieve them from paying overtime when you DO work over 40 hours per week.

The shifts are 8.15 hour shifts (ie; 2:45 p.m - 11:00 p.m)
its also a municipal job

Are you working under a collective bargaining agreement?

yes, there is a collective bargaining agreement, Im not sure of the details as the first year is generally a probationary status for employees.

CBAs are most always very specific about probationary periods and working conditions such as workweeks, schedules, overtime, etc..

I suggest you read it carefully and contact your union representative to seek some guidance.

I am not allowed to read it because i was told probationary employees cannot benefit from anything provided by the collective bargaining agreement. They are also not allowed to be in the union until after probation