Languages

he 4 main South Indian languages are individual languages on their right due to centuries of independent evolution. But, they do have a lot of closeness. The similarities are especially stronger in the east-west axis.

For instance, Malayalam and Tamil share a lot of common culture and shared vocabulary. Both the scripts evolved from the Vatteluttu alphabet. The main difference is that Malayalam has much more Sanskrit influence than Tamil. Thus, a Tamil could easily get by in Kochi/Ernakulam and in the same way a Malayali can get by in southern TN quite easily. Tamil and Malayalam are probably more closer than Bhojpuri & Bengali or Punjabi & Kashmiri.

Kannada and Telugu share even more similarities than other south Indian language pairs.  The script for instance is very close and some vocabulary is shared. They are probably more closer than Hindi & Bhojpuri. One reason was that they often shared rulers. On other hand, present day Karnataka & Tamil Nadu often didn't share rulers. 

Tamil and Telugu do share cultural and linguistic similarities. I was often forced to watch Telugu movies with my family [my mom and aunt can speak/understand Telugu well] and found it quite easy to understand. One Tamil comedy scene involves a comedian saying the only difference between these two languages is the presence of "lu", "du" at the end for Telugu. Of course, it is not that similar, but there exists common vocabulary. 

All the 4 belong to the same language family and see how they are all related:

Some Dravidian languages in different parts of the subcontinent.