Can we use Piti as a meditation subject? Can we use Piti as a meditation subject?
There are 40 meditation objects are given in Visuddhimagga as follows.
Ten recollections; ten foul objects; ten kasinas; four sublime abidings; four formless absorptions; one resolution into elements; and one perception of the filthiness of food.
My question is whether we can keep four form absorptions as a meditation objects, but specifically Piti as a meditation object.
Anything can be used as an object of meditation.  Hell, you could use a chocolate cake if it tickled your fancy.  But just because you can use anything, it doesn't mean that you should.  The best objects are those that are consistent, stable, repeatedly accessible, and don't give rise to additional mental formations.  The breath is most often used because it checks all of those boxes quite easily.  Piti can meet those same requirements, but until you hit the 1st jhana, it is entirely too unstable and inconsistent.  It can also give rise to clinging and attachment which would ultimately prevent you from moving past the 2nd jhana.