Question: What is the prime reason for the 4th to be known as the root of enlightenment?
Technically, because it is the beginning of the moksha trikona.
Practically, because it is the inner self, and enlightenment cannot proceed or even start without some degree of self-realization.
Question: What makes a person having a positive 4th house interested in bhakthi?
It is not sure that a positive 4th house will generate interest in bhakti-yoga (spiritual devotion). The rest of the chart has to have spiritual inclinations for the 4th house to produce its spiritual effect: bhakti.
The 4th house deals with bhakti because it is the inner core of ourself – it is our inner, private, emotions. Bhakti, devotion, is an internal emotion. It is the most sublime internal emotion: love.
The 4th house directly affects happiness because it is the seat of our emotions. Love (bhakti) brings the highest happiness (ananda, prema-ananda).
– Vic DiCara