I get it. But the original question itself is if 4 of pentacles as a type of relationship could mean a secret affair or to simplify it does 4 of pentacles have secretiveness as one of its meanings given the fact that high priestess clarified it.
Or if you could come up with another meaning of 4 of pentacles that could be reinforced with high priestess.
I think it is important to consider that he is the suit of Pentacles, which is about being practical, dependable, etc. Similar to the 4 of Swords and the 4 of Cups, he is "cutting himself off"..... Fours do not imply a "threesome." If you do not consider a "threesome" as cheating....that is to say, nobody is bothered by it, I would have to ask the question, "if there's nothing wrong going on, why can't you talk about it." The High Priestess might be asking such a question. In other words, she is saying, "if you ask the question properly, you will get a proper answer." On the other hand, being a 4 and not a 3, I would not call it an affair with a 3rd person. In fact, the 4 in numerology is actually a 1 (add up the numbers 1+2+3+4=10=1). So I would say the card is about isolation..and the various facets to be explored in that perspective