How to check whether a data type can be cast to another

Hello all,

I am trying to write a function that takes one source and one target
data type identifiers, each as a '/text/' variable, as arguments and
return true if the cast can be made, false otherwise.

I consulted the pg_cast and pg_type tables, however, I found no direct
casts from 'int4' to 'text'. Even though, you SELECT CAST(4::int4 AS
text) The possibility of indirect casts that use multiple intermediate
casts is slim - to me. Is an 'int4' cast to 'text' by means of multiple
intermediate casts from 'int4' to 'char' and eventually to 'text'?

I would prefer to have a function written in PG/PLSQL for now as we are
prototyping the system. Could someone help me write such a function,
please?

Thank you in advance.