Here are the dictionary main entries: reckon 1. to count, compute, or calculate, as in number or amount. 2. to esteem or consider; regard as: to be reckoned an authority in the field. 3. (Regional Chiefly Midland and Southern U.S. ) to think or suppose.
suppose 1. to assume (something), as for the sake of argument or as part of a proposition or theory: Suppose the d
The four mean basically the same thing, but there is a slight shade in meaning. They all mean that the speaker is less than absolutely certain that what he says is true:
1. "I think he's coming to the party." You are not certain that he's coming, and so you say this, rather than something like, "He's coming to the party.", which expresses certainty that he's coming.