The format is confusing a lot of people. It doesn't matter where the teams finish in Pool A. Bangladesh will definitely be playing their quarter final against whoever finishes 1st in Pool B.
As hosts, Australia and New Zealand are A1 and A2, and the next two highest teams in the ODI rankings were made A3 and A4. This was Sri Lanka and England. With England knocked out, with Bangladesh going through in their place they take A4. Who the teams from Pool A in the quarter finals
will be determined by who finishes where in Pool B. This means the quarter finals will be:
Wednesday 18th March: Sri Lanka (A3) v 2nd in Pool B, Sydney
Thursday 19th March: Bangladesh (A4) v 1st in Pool B, Melbourne
Friday 20th March: Australia (A1) v 4th in Pool B, Adelaide
Saturday 21st March: New Zealand (A2) v 3rd in Pool B, Wellington
It is explained here:
Finals - Fixtures | Cricket Fixtures - World Cup 2015 | Official Website