What are the differences between 'I think', 'I suppose', 'I guess', and 'I reckon'? In which case is one more preferred than the others?
Much depends on context. The most important thing to remember is that by and large where the same general sense is intended, guess and reckon are very much more informal (suitable for casual spoken contexts, but not really for anything remotely formal). But there can also be other nuances...
In a context like that, think may simply be a "neutral" verb indicating that's what the speaker believes to be the case. But suppose can often convey a certain "resigned acceptance" of something which is implicitly assumed to be true.
That same "resigned acceptance" sense can also apply with informal guess. So, for example,...
In that context, (b) and (c) convey limited enthusiasm for the idea, where (d) is usually more "positive".
I'm sure not everyone will agree, but I personally find that of the "informal" versions, reckon is somewhat more dialectal/rustic/folksy than guess.
In recognition of @Tyler's now-deleted answer, note that there are contexts where the choice of verb may reflect degree of certainty. For example, as a response to "I wonder why geese migrate"...
Remember that just because think/suppose are more appropriate in formal contexts doesn't imply they're less appropriate in informal contexts. In the above (informal) context, think implies the speaker already had this opinion (possibly with near certainty), so the statement carries more conviction. On the other hand, guess carries more the implication I don't know, but if I had to guess, this is what I might think (i.e. - the "reluctance" referred to earlier applies to willingness to express an opinion at all here).