Now would be a great time to do it. Have you also repotted and checked for spider mites, and to see if it's rootbound? Is it getting lots of light all day?

I think it's irresponsible to suggest that now is a good time to do such radical work on the tree w/o knowing more about it. Any time you defoliate a tree or branch, it severely weakens it, so if the tree is not growing with excellent vitality now, it's simply dangerous, even though the plant is genetically very vigorous. For instance: If the tree is being kept indoors & not under excellent lighting, it's almost surely not a candidate for the radical reduction. If the tree is outdoors and you don't have excellent supplemental lighting to bring the tree into in Sep, I'd still forgo the radical reduction.

I would first remove branches that spoil the tree's form or are not being left for other than aesthetic reasons (sacrifice branches eg). I would then thin the tree by first going through & eliminating those areas where there are several branches emanating from the same point (very common in this Ficus). After that, I would shorten branches so that a leaf or two and possibly some new buds remain on the branch. All this will allow more light and air circulation to stimulate back-budding on the interior of the tree.

This procedure isn't dangerous & you can allow the tree to recover until next June - a better time to undertake what you propose. By then, back-budding will have produced lots of new growth, presenting plenty of new opportunities to shorten branches. If you cut the tree back hard now in zone 4, you have only about a month for the tree to make enough mature foliage so the canopy is an energy producer instead of an energy sink. If a plant is not producing more energy than it's using, it's dying.

In short - how far you can cut this tree back depends on how much stored energy the tree has now, but you also need to consider the weakening effects and the fact that you may be putting the tree at a severe energy disadvantage (unable to defend against insect/disease) over a long winter.

Al

Thanks for these responses. The tree spends summer outdoors & winter in a southern window & it has seemed healthy the last three years under these conditions. I repotted it last year. I think I will do some modest trimming back at this point, then a more radical restructuring next summer.