I wouldnt risk combining at this point. If the dwindling hive has a high mite count or another cause then you risk wiping out all the bees with the combine. Better to get a healthy package of bees then keeping around a sickly hive into the new year.
400 bees is about 1/4 of 1 comb. I like to see about 4 frames minimum solid with bees. Fewer than that, the odds are very bad. I get real winter, which is more pressure, but even so it sounds like there were cues earlier in the year that would have shown trouble in time to address things.
Bees are very different from mammals or birds, and before you say "duh!", just keep in mind that in early fall, a starving mammal or bird can be fed and will probably be OK by winter. Not so for bees - in late summer/early fall, the hive needs to be populous enough to "pamper" the new bees being born. Those are called the winter bees. If their sisters are able to forage and to feed them so they don't have to make many flights or become nurse bees, then they will be able to build their fat stores for the winter. If those winter bees have to forage early, or feed younger sisters, they will not be able to build fat stores.
And if the varroa mites are too populous, the fat stores are drained early and those bees won't be able to make it though winter.
fleas and worms aren't nearly as hard on their prey!