Right now I have plants in two round 4 inch diameter pots, I would like to transfer these to square pots and would like to know (since it says that plant should repotted in pots one size bigger than they are in now) what size would be equivalent in the square pots that would be the next size? I understand from people that pots that are square are usually bigger than their round counterparts is this true?

The square pots hold more soil than the round, so yes they are larger, by volume. It would help to know what kind of plants you want to pot-up. Some like to be a bit rootbound and others want more room to expand. And some don't seem to care, they just love to grow. Pictures are always helpful, too. to NGA.

Stan has two threads going on the same subject:

The thread "Indoor Plant pot sizes" in Ask a Question forum