Q.How do I encourage hostas to spread?
We planted variegated hostas in our re-landscaped backyard about 4 years ago, spacing them 2- 3 feet apart. They all seem healthy, in tall stands with large leaves and few pest problems, but we want them to spread– to all merge together, not sit in clumps. “Too much spreading” is the only hosta problem I can find when I try to research this, so what are we doing wrong? They are on a well-watered gradual slope in medium shade with no apparent problems, short of girth. Do they spread more as they get older, or is spreading not a feature of variegated hostas?

There are well over 2,500 different hosta cultivators.
The growing patterns may very from each variety.
I don't think your doing anything wrong. Over time the hosta will become larger and close in the space.
You could plant additional plants in the open areas if you do not want to wait.
Here is a link about the growing requirements.
https://www.gardeningknowhow.com/ornamental/foliage/hosta/growing-hosta-plants.htm