Have a few doubts related to Divya Prabandha vyAkhyAna: What are nAlAyira(4000)-paDi, ArAyira (6000)-paDi? What was the basis for these categorizations? What are the differences between these? Which one must we learn first? Is there a sequence or order of learning? Where are these texts available? Are these each different explanations of Divya Prabandham? Or are they expansion of the meanings?

A Grantham is made up of 32 syllables. nAlAyira (4000)-paDi includes 4000 such Grantham-s. ArAyira (6000)-paDi includes 6000 Grantham-s.
These are approximate categorizations based on the syllables.
nAlAyira (4000)-pAdi is abbreviated; ArAyira (6000)-paDi is expanded.
There is no sequence to learn this. What are expositions that one has an affinity for, they can learn that. In our tradition ArAyira (6000)-paDi is learned first.
There are several publications for Divya Prabandham vyAkhyAnam. ArAyira (6000)-paDi commentary has been published by Andavan Ashramam. Periyavachan Pillai commentary is available at Rangavilas Mandapam at Srirangam. Another publication is available for nAlAyira (4000)-paDi vyAkhyAnam by Uttamur Svami.
There could be minor differences based on the publication.