The usual instrumentation in a piano quartet are:

* Piano * Violin * Viola * Cello

Violin + Cello + Piano

Violin + Violin + Piano

Violin + Viola + Piano

That is correct! The piano (normally) is not included in the "trio" or "quartet" count.

Kegelstatt Trio was created in 1786.

Roque Cordero has written:

'Sonata breve, for piano' -- subject(s): Sonatas (Piano)

'String quartet, no. 1, for 2 violins, viola and 'cello' -- subject(s): String quartets, Scores and parts

'Quinteto, flor flute, B [flat] clarinet, violin, violoncello, and piano' -- subject(s): Quintets (Piano, clarinet, flute, violin, violoncello)

'Tres mensajes breves, for viola and piano' -- subject(s): Viola and piano music