It is absolutely nobody else's business what is inside a 4 year old's pants.
Has anyone told the child this? Seeing as the child is showing the contents of said pants to other children?
Also, may I just highlight the absurdity of the sentence "nobody else's business" on a parenting site, especially when followed with speculation that the child might have one of a range of extremely rare medical conditions, all of which would need management and continued follow-up from specialists?
Even if your child does not have a complex medical condition, parenting is jusr continuous management of "what is inside a child's pants" from birth onwards. It does not stop at four, unless you are an irresponsible ratbag masquerading as a parent!
Sure, the majority of four-year-olds are potty-trained, and so presumably going to the toilet independently, but even for them, occasional accidents are normal, needing support from an adult. Kids of that age still don't necessarily wipe properly. Then there's the urinary tract infections, the constipation, and discovering that all the children in the class have worms from shared playdough.
"Nobody else's business" indeed!