These aren’t mutually exclusive ideas. While four may be too few, 40 could easily be too many. And the more domains a person has, the more relationships might make sense. At the same time, for people with one key domain, anything over ten relationships is probably clutter. Six to eight might make the most sense. The balance between protecting yourself and not sending people to places where you get little back is a balancing act that you have to fine tune, well, basically forever.
Also, you won’t die if you experiment some. Cut back to fewer a couple months, see how much that improves your bottom line. If it improves a LOT, then the risk to reward of fewer relationships might outweigh the safety benefits of more partners. On the other hand if you make a trivial amount more, then safety will probably seem a better idea.