I'm going to go out on a limb here and state something that I feel probably will go against the grain for some people. The idea of "The Fab Four" being an united front and showing "togetherness" within the royal family, to me, seems like it could be billed as "WilliamandHarry, Diana's boys with wives: The Sequel". This isn't how business is conducted.
There are incentives that both couples are working on together. Heads Together is one and all four of the "The Fab Four" have taken this incentive into their own personal incentives for mental health. The British monarchy, in my opinion, is evolving as it prepares to transition between reigns. Things *are* changing. William and Harry are now full time senior working royals. One is preparing to step into the role of The Duke of Cornwall (and Cambridge) and perhaps The Prince of Wales and eventually will be monarch, himself. Harry is finding his own niche and it makes sense for him to "branch out", create his own foundation and even move his offices to Buckingham Palace. It makes sense as once Charles is King, financing the offices of William and of Harry will be done in separate ways. The "Firm" is the parent company with subsidiaries so to speak.
As the reach of the "Firm" is far and wide and when you throw in the Commonwealth nations, charities and organizations that are on the global level and only so many days in year, to cover more area, it makes sense to work more as individuals than as a "team". Really, when we look at the CC and the engagements listed, most are work of the individual royal. This was even demonstrated in the recent South African tour. Harry did this and Meghan did that.
Grouping four people together for events or engagements just to paint a picture of "unity" would probably mean that less would be done at that time would, to me, scratch individual events and engagements that could have happened.
We'll see the British Royal Family en masse as we usually do. On family occasions such as walking to church on Christmas and Trooping the Color on the balcony and such. When it comes to the "Firm" though and the work the royals do, that's a horse of a different color. Its the family business.