"When I was a child, I spake as a child, I understood as a child, I thought as a child."
1 Corinthians 13:11
Obviously, if a reader can’t tell which character is having which thoughts, then the reader will be drawn out of the story in a confusing effort to determine what’s happening. This is a most damaging type of alienation (anything that reminds the reader that she is reading a story) and doing this can cause the reader to throw the book against the wall. (Fortunately, book throwing happens much less often given the growing popularity of eBooks.)
In this case, under the governing parent rule, the writer could employ four different POVs in the same tightly written scene. This is not really a case of breaking the ‘child’ rule against using more than one POV in a scene because a parent rule always has supremacy over child rules.
In order to use four different POVs, one after the other, in one scene the author would have make it crystal clear which thoughts belong to which character. Given the situation in the above example, making each POV change crystal clear should not present a major problem to the experienced writer. Indeed, the context of the above example almost demands such a POV-changing approach.
To sum it up, it is possible, by following higher order parent rules, to bypass child rules without becoming a ‘rule breaking’ outlaw writer or a prima donna of divine dispensation from the rules that mere mortal writers must obey.
Caveat: It is always a good policy to follow the ‘child’ one-POV-per-scene rule. The problem with following the parent rule, especially for experienced writers who are familiar with their major characters’ speech patterns, is this: what is perfectly clear to the author may be as clear as mud to their readers. When several POV changes are made in a scene the result needs to be proof read by a reader who knows nothing about the story a reader would not know at that point.