What was Buddha's purpose in constructing The Four Stages of Enlightenment?
The four stages define what the basis of enlightenment is, namely, losing the belief in an inherent self ("stream-entry") ; and, at the same time, further define what full enlightenment is ("arahant"), namely, the complete uprooting of greed, hatred &amp; delusion. Therefore, if you or a teacher (guru) still has lust, hatred or selfishness then this means full enlightenment has not been reached. 

For example, there are many intellectuals (e.g. Sam Harris) who speak and act to give an impression they are enlightened because they speak theories about "no self". However, if we examine their other views and beliefs, we can work out if they are really enlightened or not. 

Or there is a certain chat site where members claim to practise "jhana" yet they also post about their sexual lusts. Since a master of jhana will be at least a once-returner but often a non-returner, any significant lust should not arise in a once-returner or non-returner. 

The same applies to us. The four stages are a guide to us of the level of (our) enlightenment. They are not for boasting but for the opposite. They are so we do not overestimate our experience.