Using Solr/Lucene for searching non-text tables?
Will/can Solr/Lucene searches be faster than PostgreSQL even if no
  full-text search is involved?


Yes. As per your quoted example, it can be many times faster than a relational database for certain use cases. Not surprising really.

Solr is a search engine. PostgreSQL is a relational database engine.

Solr is built from the ground up to do one thing well, search. It cannot do the work of a transactional database system. Solr cannot guarantee ACID compliance, it does not efficiently manage concurrency, it is not a viable candidate for OLTP workloads.

PostgreSQL is built from the ground up to do one thing well, transactional processing. It can also do a reasonable job of providing search type facilities but that is not its primary role. It does guarantee ACID compliance, it does manage concurrency, it is not a search engine.


  The 4m rows only change once a year.


In which case you probably don't need the key features of a relational database engine. 

You need a search engine. Could be Solr, Lucene, ElasticSearch, Amazon Cloud Search or one of a dozens others but first and foremost it will be a search platform, not a relational database.