John Rumm brought next idea :
You can check by opening task manager when you have open a "typical" set of
applications, and clicking on the Performance tab, and then the "Memory"
entry on the left. Look at the "Committed" amount at the bottom of the
window. If that is less than the physical ram, then there will be no
particular gain in having more.

That shows 4.1 committed of 8.8 Gb - I don't know where the extra .8Gb
came from, but it seems not worthwhile adding more. Thanks..