The Nalvar represent the four relationships a person can develop in his bhakthi towards god.
1. Appar - Dasa ( servant) bhava - Here the relationship is master and servant. The life of appar is full of this bhava. His songs also sysmbolises that.
2. Sambandar ( son) putra bhava - Here the relationship is father and son. Sambandar also sings like "Anai Namade, enadurai thanadurai aga"
3. Sundarar ( Friend) Saha Bhava - The life of sundarar is represents this friend ship. God even goes for thoothu. He can sing Vazhndu podere. ( sundarar uses the phrase meela adimai very sarcastically noting the way god came to him first) but his relationship with god was full of friendship
4. manikkavasagar ( student) sishya bhavam. The relationship is teacher and student.
They showed the 4 ways to us through their life, their relationship with god and their songs.
the 4 samayakkuravars bhakthi bhavams was expalined by Kanchi Mahaswami . I am quoting this from memory as i am not having the book with me now.( I have just given the songs that came to my mind and any mistake in this will be mine )