Other thoughts
Intreguing thread, Wind. Here's a few more thoughts (some of them riffing off what others have already said) about how Pents might relate to relationships:
1) In a relationship spread, Pents could be reflecting on the health/luck of the relationship. Is it healthy, is it strong, is it rich, is it lucky?
2) Pents could reflect on a more pragmatic relationship. If both people are pragmatic and down to earth, not romantic, then they might view their partnership more like a business relationship. A give and take of services and riches, working together to make a home and finance themselves. This would make some couples very content, as compared to romance, or passion which, they might feel, was a waste of time. To each their own.
For example, 9/Pents might indicate a boyfriend/girlfriend who offers no passion or romance, but who is safe, comfortable, well off and well suited to the person in question. If the querent wants domestic bliss most in life, then this is probably their guy/girl. If, on the other hand, they need romance or passion or intellectual stimulation...well, then they're out of luck. This is a card about someone solid as a rock and able to comfortably provide.
I might even say, if I was reading the cards for a well-off man, that this was a trophy wife. Something to beautify his home and show off, a marriage of convience and mutually beneficial.
3) Alternately, Pents might indicate trouble in a relationship because one person views the other as "property" rather than as a person. We all have those jealous moments when we say, "That's MY man/woman!" and get possessive. I think 4/Pents could very well indicate this at its worse. The person who won't let his wife/her husband have friends or a life outside because they view them as a possession.
4) And, of course, Pents could be commenting on the physical relationship the couple is having. How is the sex? It might not be inventive or passionate like it would be if there were wands, but with Pents could indicate that it's healthy and thriving...or, with, say, 7/pents, that the couple is having a lull on that score and need to be patient.
Just a few more things to think about.