It could be hardware disease, get a cow magnet and see if that helps. Unless he had chronic lung issues I would doudt it being heart failure at 4 y/o.
If it is heart fialure, there is nothing to cure it but death.
Yes good chance hardware but Google St George Disease (Pimelea poisoning). It is specific to an area in Australia where the plant grows but symptoms are as for congestive heart failure, you never know, he might have a taste for a similar plant.